Method and device for encoding intra prediction mode for image prediction unit, and method and device for decoding intra prediction mode for image prediction unit

ABSTRACT

Methods and apparatuses for encoding and decoding an intra prediction mode of a prediction unit of a chrominance component based on an intra prediction mode of a prediction unit of a luminance component are provided. When an intra prediction mode of a prediction unit of a luminance component is the same as an intra prediction mode in an intra prediction mode candidate group of a prediction unit of a chrominance component, reconstructing the intra prediction mode candidate group of the prediction unit of the chrominance component by excluding or replacing an intra prediction mode of the prediction unit of the chrominance component which is same as an intra prediction mode of the prediction unit of the luminance component from the intra prediction mode candidate group, and encoding the intra prediction mode of the prediction unit of the chrominance component by using the reconstructed intra prediction mode candidate group.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/003,946, filed on Jun. 8, 2018, in the U.S. Patent and TrademarkOffice, which is a continuation of U.S. patent application Ser. No.13/997,524, filed on Jun. 24, 2013, in the U.S. Patent and TrademarkOffice, which is a National Stage of International Patent ApplicationNo. PCT/KR2011/010066, filed on Dec. 23, 2011, which claims priorityfrom U.S. Provisional Patent Application No. 61/426,684, filed on Dec.23, 2010, in the U.S. Patent and Trademark Office, the disclosures ofwhich are incorporated herein in their entireties by reference.

BACKGROUND 1. Field

Methods and apparatuses consistent with the exemplary embodiments relateto encoding and decoding of an image, and more particularly, to encodingan intra prediction mode of an image prediction unit which efficientlyencodes an intra prediction mode of a chrominance component based on acorrelation between an intra prediction mode determined for an imageprediction unit of a luminance component and the intra prediction modeof the chrominance component, and decoding an intra prediction mode ofan image prediction unit.

2. Related Art

In an image compression method, such as Moving Picture Experts Group(MPEG)-1, MPEG-2, MPEG-4, or H.264/MPEG-4 Advanced Video Coding (AVC), apicture is divided into macroblocks in order to encode an image. Each ofthe macroblocks is encoded in all encoding modes that can be used ininter prediction or intra prediction, and then is encoded in an encodingmode that is selected according to a bit rate used to encode themacroblock and a distortion degree of a decoded macroblock based on theoriginal macroblock.

As hardware for reproducing and storing high resolution or high qualityvideo content is being developed and supplied, there is an increasingneed for a video codec which effectively encodes or decodes the highresolution or high quality video content. In a related art video codec,a video is encoded using a limited prediction mode based on macroblockseach having a predetermined size.

SUMMARY

The exemplary embodiments provide methods and apparatuses forefficiently encoding and decoding an intra prediction mode of achrominance image component prediction unit corresponding to a luminanceimage component prediction unit based on an intra prediction modedetermined for the luminance image component prediction unit.

According to an aspect of the exemplary embodiments, an intra predictionmode of a prediction unit of a chrominance component is efficientlyencoded and decoded by reconstructing an intra prediction mode candidategroup applied to the prediction unit of the chrominance componentaccording to whether an intra prediction mode of a luminance componentis the same as the intra prediction mode of the chrominance component.

According to the exemplary embodiments, redundancy between an intraprediction mode of a prediction unit of a luminance image component andan intra prediction mode of a prediction unit of a chrominance imagecomponent may be removed, and the intra prediction mode of theprediction unit of the chrominance image component may be efficientlyexpressed based on the intra prediction mode of the prediction unit ofthe luminance image component. Also, the number of bits used to signalthe intra prediction mode of the prediction unit of the chrominanceimage component may be reduced.

According to an aspect of the exemplary embodiments, there is provided amethod of encoding an intra prediction mode of an image, the methodcomprising: determining an intra prediction mode of a prediction unit ofa first image component constituting the image by using a first intraprediction mode candidate group comprising a plurality of intraprediction modes; determining an intra prediction mode of a predictionunit of a second image component corresponding to the prediction unit ofthe first image component by using a second intra prediction modecandidate group comprising a plurality of intra prediction modes; whenthe determined intra prediction mode of the prediction unit of the firstimage component is the same as one of predetermined intra predictionmodes included in the second intra prediction mode candidate group,reconstructing the second intra prediction mode candidate group toremove redundancy of an intra prediction mode which is included in thesecond intra prediction mode candidate group and is same as thedetermined intra prediction mode of the prediction unit of the firstimage component; and encoding the determined intra prediction mode ofthe prediction unit of the second image component based on thereconstructed second intra prediction mode candidate group.

According to another aspect of the exemplary embodiments, there isprovided an apparatus for encoding an intra prediction mode of an image,the apparatus including: a first intra prediction mode determiner thatdetermines an intra prediction mode of a prediction unit of a firstimage component constituting the image by using a first intra predictionmode candidate group including a plurality of intra prediction modes; asecond intra prediction mode determiner that determines an intraprediction mode of a prediction unit of a second image componentcorresponding to the prediction unit of the first image component byusing a second intra prediction mode candidate group including aplurality of intra prediction modes; and an intra prediction modeencoder that when the determined intra prediction mode of the predictionunit of the first image component is the same as one of predeterminedintra prediction modes included in the second intra prediction modecandidate group, reconstructs the second intra prediction mode candidategroup to remove redundancy of an intra prediction mode included in thesecond intra prediction mode candidate group which is same as thedetermined intra prediction mode of the prediction unit of the firstimage component, and encodes the determined intra prediction mode of theprediction unit of the second image component based on the reconstructedintra prediction mode candidate group.

According to another aspect of the exemplary embodiments, there isprovided a method of decoding an intra prediction mode of an image, themethod comprising: obtaining intra prediction mode information of aprediction unit of a first image component constituting the image from abitstream; when an intra prediction mode of the prediction unit of thefirst image component is the same as one of predetermined intraprediction modes included in a second intra prediction mode candidategroup applicable to a prediction unit of a second image component,reconstructing the second intra prediction mode candidate group toremove redundancy of an intra prediction mode included in the secondintra prediction mode candidate group which is same as the determinedintra prediction mode of the prediction unit of the first imagecomponent; and decoding an intra prediction mode of the prediction unitof the second image component based on the reconstructed second intraprediction mode candidate group.

According to another aspect of the exemplary embodiments, there isprovided an apparatus for decoding an intra prediction mode of an image,the apparatus comprising: an intra prediction mode obtainer that obtainsintra prediction mode information of a prediction unit of a first imagecomponent constituting the image from a bitstream, when it is determinedthat an intra prediction mode of the prediction unit of the first imagecomponent is the same as one of predetermined intra prediction modesincluded in a second intra prediction mode candidate group applicable toa prediction unit of a second image component, reconstructs the secondintra prediction mode candidate group to remove redundancy of an intraprediction mode which is included in the second intra prediction modecandidate group and is same as the determined intra prediction mode ofthe prediction unit of the first image component, and determines anintra prediction mode of the prediction unit of the second imagecomponent based on the reconstructed second intra prediction modecandidate group; a first intra prediction performer that performs intraprediction on the prediction unit of the first image component based onthe intra prediction mode of the prediction unit of the first imagecomponent which is obtained from the bitstream; and a second intraprediction performer that performs intra prediction on the predictionunit of the second image component based on the determined intraprediction mode of the prediction unit of the second image component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an apparatus for encoding avideo, according to an exemplary embodiment.

FIG. 2 is a block diagram illustrating an apparatus for decoding avideo, according to an exemplary embodiment.

FIG. 3 is a diagram for describing a concept of coding units, accordingto an exemplary embodiment.

FIG. 4 is a block diagram illustrating an image encoder based on acoding unit, according to an exemplary embodiment.

FIG. 5 is a block diagram illustrating an image decoder based on acoding unit, according to an exemplary embodiment.

FIG. 6 is a diagram illustrating deeper coding units according to depthsand partitions, according to an exemplary embodiment.

FIG. 7 is a diagram for describing a relationship between a coding unitand a transformation unit, according to an exemplary embodiment.

FIG. 8 is a diagram for describing information of coding units accordingto depths, according to an exemplary embodiment.

FIG. 9 is a diagram illustrating coding units according to depths,according to an exemplary embodiment.

FIGS. 10, 11, and 12 are diagrams for describing a relationship betweencoding units, prediction units, and frequency transformation units,according to an exemplary embodiment.

FIG. 13 is a diagram illustrating a relationship between a coding unit,a prediction unit, and a transformation unit according to encoding modeinformation of Table 1, according to an exemplary embodiment.

FIG. 14A through FIG. 14C are diagrams illustrating formats ofprediction units of a luminance component and a chrominance component,according to exemplary embodiments.

FIG. 15 is a table illustrating the number of intra prediction modesaccording to sizes of luminance component prediction units, according toan exemplary embodiment.

FIG. 16A is a diagram showing an intra prediction mode applicable aluminance component prediction unit having a predetermined size,according to an exemplary embodiment.

FIG. 16B is a diagram illustrating directions of the intra predictionmodes of FIG. 16A.

FIG. 16C is a diagram for describing a method of performing intraprediction on a luminance component prediction unit by using the intraprediction modes of FIG. 16A, according to an exemplary embodiment.

FIG. 17 is a diagram for describing an intra prediction mode applied toa luminance component prediction unit having a predetermined size,according to another exemplary embodiment.

FIGS. 18A through 18C are reference diagrams for describing intraprediction modes having different directionalities, according to anexemplary embodiment.

FIG. 19 is a reference diagram for describing a bi-linear mode accordingto an exemplary embodiment.

FIG. 20 is a graph which illustrates a relationship between a currentpixel and a peripheral pixel located on a line having a directionalityof (dx, dy), according to an exemplary embodiment.

FIG. 21 is a graph illustrating a change in a neighboring pixel locatedon a line having a directionality of (dx, dy) according to a location ofthe current pixel, according to an exemplary embodiment.

FIGS. 22 and 23 are diagrams for describing a method of determining anintra prediction mode direction, according to an exemplary embodiment.

FIG. 24 is a reference diagram for describing an available intraprediction mode according to an exemplary embodiment.

FIGS. 25A and 25B are reference diagrams for describing an intraprediction mode according to another exemplary embodiment.

FIG. 26 is a block diagram illustrating an apparatus for encoding anintra prediction mode of an image, according to an exemplary embodiment.

FIG. 27 is a flowchart illustrating a method of encoding an intraprediction mode of an image, according to an exemplary embodiment.

FIG. 28 is a flowchart illustrating a process of encoding an intraprediction mode of an image, according to an exemplary embodiment.

FIG. 29 is a block diagram illustrating an apparatus for decoding anintra prediction mode of an image, according to an exemplary embodiment.

FIG. 30 is a flowchart illustrating a method of decoding an intraprediction mode of an image, according to an exemplary embodiment.

FIG. 31 is a flowchart illustrating a process of decoding an intraprediction mode of an image, according to an exemplary embodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The exemplary embodiments will now be described more fully withreference to the accompanying drawings, in which exemplary embodimentsare shown.

FIG. 1 is a block diagram illustrating an apparatus 100 for encoding avideo, according to an exemplary embodiment.

The apparatus 100 includes a maximum coding unit splitter 110, a codingunit determiner 120, and an output 130.

The maximum coding unit splitter 110 may split a current picture basedon a maximum coding unit which is a coding unit having a maximum sizefor the current picture of an image. If the current picture is largerthan the maximum coding unit, image data of the current picture may besplit into at least one maximum coding unit. The maximum coding unit maybe a data unit having a size of 32×32, 64×64, 128×128, or 256×256, and ashape of the data unit may be a square having a width and length insquares of 2 greater than 8. The image data may be output to the codingunit determiner 120 according to the at least one maximum coding unit.

A coding unit may be characterized by a maximum size and a depth. Thedepth denotes the number of times the coding unit is spatially splitfrom the maximum coding unit, and as the depth deepens, deeper codingunits according to depths may be split from the maximum coding unit to aminimum coding unit. A depth of the maximum coding unit is an uppermostdepth and a depth of the minimum coding unit is a lowermost depth. Sincea size of a coding unit corresponding to each depth decreases as thedepth of the maximum coding unit deepens, a coding unit corresponding toan upper depth may include a plurality of coding units corresponding tolower depths.

As described above, the image data of the current picture is split intothe maximum coding units according to a maximum size of the coding unit,and each of the maximum coding units may include deeper coding unitsthat are split according to depths. Since the maximum coding unit issplit according to depths, the image data of a spatial domain includedin the maximum coding unit may be hierarchically classified according todepths.

A maximum depth and a maximum size of a coding unit, which limit thetotal number of times a height and a width of the maximum coding unitare hierarchically split, may be preset.

The coding unit determiner 120 encodes at least one split regionobtained by splitting a region of the maximum coding unit according todepths and determines a depth in which final encoding results are to beoutput according to the at least one split region. That is, the codingunit determiner 120 encodes the image data according to deeper codingunits according to depths for each maximum coding unit of the currentpicture, and selects a depth having a least encoding error as a codeddepth. The determined coded depth and the image data according tomaximum coding units are output to the output 130.

The image data in the maximum coding unit is encoded based on the deepercoding units corresponding to at least one depth equal to or below themaximum depth, and results of encoding the image data are compared basedon each of the deeper coding units. A depth having the least encodingerror may be selected after comparing encoding errors of the deepercoding units. At least one coded depth may be selected for each maximumcoding unit.

The size of the maximum coding unit that is split as a coding unit ishierarchically split according to depths, and as the number of codingunits increases. Also, even if coding units correspond to the same depthin one maximum coding unit, it is determined whether to split each ofthe coding units corresponding to the same depth to a lower depth bymeasuring an encoding error of the image data of each coding unit,separately. Accordingly, even when image data is included in one maximumcoding unit, since the encoding errors may differ according to regionsin the one maximum coding unit, the coded depths may differ according toregions in the image data. Thus, one or more coded depths may bedetermined in one maximum coding unit, and the image data of the maximumcoding unit may be split according to coding units of at least one codeddepth.

Accordingly, the coding unit determiner 120 may determine coding unitshaving a tree structure included in a current maximum coding unit. The‘coding units having the tree structure’ include coding units having adepth determined as a coded depth from among all deeper coding unitsincluded in the current maximum coding unit. The coding units of thecoded depth may be hierarchically determined according to depths in thesame region in the maximum coding unit, and may be independentlydetermined in other regions. Likewise, a coded depth of a current regionmay be independently determined from coded depths of other regions.

A maximum depth is an index related to the number of splits from amaximum coding unit to a minimum coding unit. A first maximum depth mayindicate the total number of splits from a maximum coding unit to aminimum coding unit. A second maximum depth may indicate the totalnumber of depth levels from the maximum coding unit to the minimumcoding unit. For example, when a depth of a maximum coding unit is 0, adepth of a coding unit obtained by splitting the maximum coding unit onetime may be set to 1, and a depth of a coding unit obtained by splittingthe maximum coding unit two times may be set to 2. In this case, when acoding unit obtained by splitting the maximum coding unit four times isa minimum coding unit, since depth levels of depths 0, 1, 2, 3, and 4exist, a first maximum depth may be set to 4 and a second maximum depthmay be set to 5.

Prediction encoding and frequency transformation may be performed on themaximum coding unit. The prediction encoding and the frequencytransformation are performed based on deeper coding units according todepths equal to or less a maximum depth for each maximum coding unit.

Since the number of deeper coding units according to depths increase asthe maximum coding unit is split according to depths, encoding includingprediction encoding and frequency transformation has to be performed onall of the deeper coding units generated as the depths deepen.Prediction encoding and frequency transformation will be explained basedon a coding unit of a current depth from among one or more maximumcoding units for convenience of explanation.

The apparatus 100 may variously select a size or a shape of a data unitfor encoding image data. In order to encode the image data, operationssuch as prediction encoding, frequency transformation, and entropyencoding are performed. The same data unit may be used for alloperations, or different data units may be used for differentoperations.

For example, the apparatus 100 may select not only a coding unit forencoding the image data, but also a data unit, which is different fromthe coding unit, in order to perform prediction encoding on the imagedata in the coding unit.

In order to perform prediction encoding in a maximum coding unit, theprediction encoding may be performed based on a coding unit of a codeddepth, that is, a coding unit which is no longer split. The coding unitwhich is no longer split and becomes a basis unit for predictionencoding is referred to as a ‘prediction unit’. A partition obtained bysplitting the prediction unit may include a prediction unit and a dataunit obtained by splitting at least one of a height and a depth of theprediction unit.

For example, when a coding unit having a size of 2N×2N (N is a positiveinteger) is no longer split, a prediction unit may have a size of 2N×2Nand a partition may have a size of 2N×2N, N×N, N×2N, or N×N. Examples ofa partition type may include symmetric partitions obtained bysymmetrically splitting a height or a width of the prediction unit,partitions obtained by asymmetrically splitting the height or the widthof the prediction unit such as 1:n or n:1, partitions obtained bygeometrically splitting the prediction unit, and partitions havingarbitrary shapes.

A prediction mode of the prediction unit may be at least one of an intramode, an inter mode, and a skip mode. For example, the intra mode andthe inter mode may be performed on the partition having a size of 2N×2N,2N×N, N×2N, or N×N. Also, the skip mode may be performed only on thepartition having a size of 2N×2N. The encoding may be independentlyperformed on one prediction unit in a coding unit and a prediction modehaving a least encoding error may be selected.

Also, the apparatus 100 may also perform frequency transformation on theimage data in a coding unit based on not only the coding unit forencoding the image data but also a data unit different from the codingunit.

In order to perform frequency transformation in the coding unit, thefrequency transformation may be performed based on a data unit having asize equal to or less than that of the coding unit. For example, thedata unit for the frequency transformation may include a data unit foran inter mode and a data unit for an intra mode.

A data unit which becomes a base of frequency transformation may bereferred to as a ‘transformation unit’. Similar to a coding unit, atransformation unit in a coding unit may be recursively split into atransformation unit having a smaller size, and residual data of thecoding unit may be split according to transformation units having a treestructure according to a transformation depth.

A transformation depth indicating the number of splits to reach thetransformation unit by splitting a height and a width of the coding unitmay also be set in the transformation unit. For example, a size of atransformation unit of a current coding unit having a size of 2N×2N is2N×2N, a transformation depth may be set to 0, when a size of atransformation unit is N×N, a transformation depth may be set to 1, andwhen a size of a transformation unit is N/2×N/2, a transformation depthmay be set to 2. That is, transformation units having a tree structuremay be set according to transformation depths.

Information about deeper coding units according to depths usesinformation about a coded depth, information related to prediction, andinformation related to frequency transformation. Accordingly, the codingunit determiner 120 may determine not only a coded depth that generatesa least encoding error, but also a partition type by which a predictionunit is split into partitions, a prediction mode according to eachprediction unit, and a size of a transformation unit for frequencytransformation.

A method of determining a partition and coding units having a treestructure of a maximum coding unit will be explained in detail withreference to FIGS. 3 through 12.

The coding unit determiner 120 may measure an encoding error of deepercoding units according to depths by using a Rate-Distortion Optimizationbased on Lagrangian multipliers.

The image data output unit 130 outputs the image data of the maximumcoding unit, which is encoded based on the at least one coded depthdetermined by the coding unit determiner 120 and information about theencoding mode according to the coded depth, in bitstreams.

The encoded image data may be results obtained by encoding residual dataof the image.

The information about the coding mode according to the coded depth mayinclude information about the coded depth, information about thepartition type in the prediction unit, information about the predictionmode, and information about a size of the transformation unit.

The information about the coded depth may be defined by using splitinformation according to each depth indicating whether encoding isperformed on coding units of a lower depth instead of a current depth.When the current depth of the current coding unit is the coded depth,since image data in the current coding unit is encoded, the splitinformation may be defined not to split the current coding unit to alower depth. Alternatively, when the current depth of the current codingunit is not the coded depth, since encoding is performed on a codingunit of a lower depth, the split information may be defined to split thecurrent coding unit to obtain coding units of a lower depth.

When the current depth is not the coded depth, encoding is performed ona coding unit which is split into coding units of a lower depth. Sinceone or more coding units of a lower depth exist in one coding unit ofthe current depth, encoding may be repeatedly performed on each codingunit of the lower depth, and thus the encoding may be recursivelyperformed for coding units of the same depth.

Since coding units having a tree structure has to be determined in onemaximum coding unit and information about at least one encoding mode hasto be determined for each coding unit of a coded depth, informationabout at least one encoding mode may be determined for one maximumcoding unit. Also, since image data of the maximum coding unit may behierarchically split according to depths and thus coded depths maydiffer according to regions, information about an encoding mode and acoded depth may be set for the image data.

Accordingly, the output unit 130 may assign encoding information about acorresponding coded depth and an encoding mode to at least one of thecoding unit, the prediction unit, and a minimum unit included in themaximum coding unit.

The minimum unit may be a square data unit having a size obtained bysplitting a minimum coding unit constituting a lowermost depth by 4, andmay be a square data unit having a maximum size which may be included inall of coding units, prediction units, and transformation units includedin the maximum coding unit.

For example, the encoding information output through the output 130 maybe classified into encoding information according to deeper coding unitsaccording to depths and encoding information according to predictionunits. The encoding information according to the deeper coding unitsaccording to depths may include information about a prediction mode andinformation about sizes of partitions. The encoding information aboutthe prediction units may include information about an estimateddirection of an inter mode, information about a reference image index ofthe inter mode, information about a motion vector, information about achrominance component of an intra mode, and information about aninterpolation method of the intra mode. Also, information about amaximum size of a coding unit which is defined according to pictures,slices, or Group of Pictures (GOPs) and information about a maximumdepth may be inserted into a header of a bitstream.

According to the apparatus 100 having a simple structure, a deepercoding unit is a coding unit obtained by dividing a height or a width ofa coding unit of an upper depth, which is one layer above, by two. Thatis, when a size of a coding unit of a current depth is 2N×2N, a size ofa coding unit of a lower depth is N×N. Also, when the coding unit of thecurrent depth has a size of 2N×2N, a size of the coding unit of thelower depth is N×N. Also, the coding unit of the current depth havingthe size of 2N×2N may include a maximum 4 of the coding unit of thelower depth.

Accordingly, the apparatus 100 may form coding units having a treestructure by determining a coding unit having an optimum shape and anoptimum size for each maximum coding unit, based on a size of themaximum coding unit and a maximum depth determined in consideration ofcharacteristics of a current picture. Also, since encoding may beperformed on each maximum coding unit by using any of various predictionmodes and frequency transformation modes, an optimum encoding mode maybe determined in consideration of characteristics of coding units havingvarious image sizes.

Thus, if an image having high resolution or a large data amount isencoded in a conventional macroblock, the number of macroblocks perpicture excessively increases. Accordingly, the number of pieces ofcompressed information generated for each macroblock increases, and thusit is difficult to transmit the compressed information and datacompression efficiency decreases. However, by using the apparatus 100,image compression efficiency may be increased since a coding unit isadjusted while considering characteristics of an image while increasinga maximum size of a coding unit while considering a size of the image.

FIG. 2 is a block diagram of an apparatus 200 for decoding a video,according to an exemplary embodiment.

Referring to FIG. 2, the apparatus 200 includes a receiver 210, an imagedata and encoding information extractor 220, and an image data decoder230. Definitions of terms such as a coding unit, a depth, a predictionunit, a transformation, unit, and information about various encodingmodes for various processes of the apparatus 200 are the same as thosedescribed with reference to FIG. 1 and the apparatus 100.

The receiver 210 receives and parses a bitstream for an encoded video.The image data and encoding information extractor 220 extracts encodedimage data for each coding unit according to coding units having a treestructure according to maximum coding units from the parsed bitstream,and outputs the extracted encoded image data to the image data decoder230. The image data and encoding information extractor 220 may extractinformation of a maximum size of a coding unit of a current picture froma header of the current picture.

Also, the image data and encoding information extractor 220 extractsinformation about a coded depth and an encoding mode for the codingunits having the tree structure according to the maximum coding unitsfrom the parsed bitstream. The extracted information about the codeddepth and the encoding mode is output to the image data decoder 230.That is, the image data decoder 230 may decode image data for eachmaximum coding unit by splitting the image data of a bit stringaccording to maximum coding units.

The information about the coded depth and the encoding mode according tothe maximum coding units may be set for one or more pieces of codeddepth information, and information about encoding modes according tocoded depths may include information about a partition type of acorresponding coding unit, information about a prediction mode, andinformation about a size of a transformation unit. Also, splitinformation according to depths may be extracted as the informationabout the coded depth.

The information about the coded depth and the encoding mode according tothe maximum coding units extracted by the image data and encodinginformation extractor 220 is information about a coded depth and anencoding mode determined to have a least encoding error by repeatedlyperforming encoding on deeper coding units according to maximum codingunits at an encoding side such as the apparatus 100. Accordingly, theapparatus 200 may restore an image by decoding data according to anencoding method that generates a least encoding error.

Since encoding information about a coded depth and an encoding mode maybe assigned to a predetermined data unit from among a correspondingcoding unit, a prediction unit, and a minimum unit, the image data andencoding information extractor 220 may extract information about a codeddepth and an encoding mode according to predetermined data units. Wheninformation about a coded depth and an encoding mode of a correspondingmaximum coding unit is recorded according to the predetermined dataunits, the predetermined data units having the same information aboutthe coded depth and the encoding mode may be derived as a data unitincluded in the same maximum coding unit.

The image data decoder 230 restores a current picture by decoding imagedata in each maximum coding unit based on the information about thecoded depth and the encoding mode according to the maximum coding units.That is, the image data decoder 230 may decode encoded image data basedon read information about a partition type, a prediction mode, and atransformation unit, for each coding unit from among coding units havinga tree structure included in each maximum coding unit. The decoding mayinclude a prediction process including intra prediction and motioncompensation, and a frequency inverse transformation process.

The image data decoder 230 may perform intra prediction or motioncompensation according to each partition and prediction mode for eachcoding unit based on information about a partition type and a predictionmode of a prediction unit of coding units according to coded depths.

Also, the image data decoder 230 may perform frequency inversetransformation according to each transformation unit in the coding unit,based on size information of a transformation unit of coding unitsaccording to coded depths, for frequency inverse transformationaccording to maximum coding units.

The image data decoder 230 may determine a coded depth of a currentmaximum coding unit by using split information according to each depth.If split information indicates that image data is no longer split in acurrent depth, the current depth is a coded depth. Accordingly, theimage data decoder 230 may decode a coding unit of the current depth forimage data of the current maximum coding unit by using information abouta partition type of a prediction unit, a prediction mode, and a size ofa transformation unit.

That is, data units containing encoding information including the samesplit information may be gathered by observing the encoding informationset assigned for a predetermined data unit from among a coding unit, aprediction unit, and a minimum unit, and the gathered data units may beconsidered to be one data unit to be decoded by the image data decoder230 in the same encoding mode.

The apparatus 200 may obtain information about a coding unit thatgenerates a least encoding error by recursively performing encoding foreach maximum coding unit and may use the information to decode a currentpicture. That is, encoded image data of coding units having a treestructure determined for each maximum coding unit may be decoded.

Accordingly, even an image has high resolution or a large amount ofdata, image data may be efficiently decoded and restored according to asize of a coding unit and an encoding mode which are adaptivelydetermined for characteristics of an image by using information about anoptimum encoding mode transmitted from an encoding side.

A method of determining coding units having a tree structure, aprediction unit, and a transformation unit will be explained in detailwith reference to FIGS. 3 through 13.

FIG. 3 is a diagram for describing a concept of hierarchical codingunits, according to an exemplary embodiment.

A size of a coding unit may be expressed in width×height, and examplesof the size may include 64×64, 32×32, 16×16, and 8×8. A coding unithaving a size of 64×64 may be split into partitions having sizes of64×64, 64×32, 32×64, and 32×32, a coding unit having a size of 32×32 maybe split into partitions having sizes of 32×32, 32×16, 16×32, and 16×16,a coding unit having a size of 16×16 may be split into partitions havingsizes of 16×16, 16×8, 8×16, and 8×8, and a coding unit having a size of8×8 may be split into partitions having sizes of 8×8, 8×4, 4×8, and 4×4.

In video data 310, a resolution is 1920×1080, a maximum size of a codingunit is 64, and a maximum depth is 2. In video data 320, a resolution is1920×1080, a maximum size of a coding unit is 64, and a maximum depth is4. In video data 330, a resolution is 352×288, a maximum size of acoding unit is 16, and a maximum depth is 2. A maximum depth in FIG. 3indicates the total number of splits from a maximum coding unit to aminimum coding unit.

If a resolution is high or a data amount is large, a maximum size of acoding unit may be large so as to not only increase encoding efficiencybut also to accurately reflect characteristics of an image. Accordingly,the maximum size of the coding unit of the video data 310 and 320 havinga higher resolution than the video data 330 may be 64.

Since the maximum depth of the video data 310 is 2, coding units 315 ofthe video data 310 may include a maximum coding unit having a long axissize of 64, and coding units having long axis sizes of 32 and 16 sincedepths are deepened by two layers by splitting the maximum coding unittwice. Meanwhile, since the maximum depth of the video data 330 is 1,coding units 335 of the video data 330 may include a maximum coding unithaving a long axis size of 16, and coding units having a long axis sizeof 8 since depths are deepened by one layer by splitting the maximumcoding unit once.

Since the maximum depth of the video data 320 is 3, coding units 325 ofthe video data 320 may include a maximum coding unit having a long axissize of 64, and coding units having long axis sizes of 32, 16, and 8since the depths are deepened by 3 layers by splitting the maximumcoding unit three times. As a depth deepens, detailed information may beprecisely expressed.

FIG. 4 is a block diagram of an image encoder 400 based on a codingunit, according to an exemplary embodiment.

The image encoder 400 performs operations of the coding unit determiner120 of the apparatus 100 to encode image data. That is, an intrapredictor 410 performs intra prediction on coding units in an intramode, from among a current frame 405, and a motion estimator 420 and amotion compensator 425 perform inter estimation and motion compensationby using a reference frame 495 and the current frame 405 of an intermode.

Data output from the intra predictor 410, the motion estimator 420, andthe motion compensator 425 is output as a quantized transformationcoefficient through a frequency transformer 430 and a quantizer 440. Thequantized transformation coefficient is restored as data in a spatialdomain through an inverse quantizer 460 and a frequency inversetransformer 470, and the restored data in the spatial domain is outputas the reference frame 495 after being post-processed through adeblocker 480 and a loop filter 490. The quantized transformationcoefficient may be output as a bitstream 455 through an entropy encoder450.

In order for the image encoder 400 to be applied to the apparatus 100,all elements of the image encoder 400, i.e., the intra predictor 410,the motion estimator 420, the motion compensator 425, the frequencytransformer 430, the quantizer 440, the entropy encoder 450, the inversequantizer 460, the inverse transformer 470, the deblocker 480, and theloop filter 490 have to perform operations based on each coding unitfrom among coding units having a tree structure in consideration of amaximum depth for each maximum coding unit.

Specifically, the intra predictor 410, the motion estimator 420, and themotion compensator 425 determine a prediction unit and partitions ofeach coding unit from among a coding units having a tree structure inconsideration of a maximum depth and a maximum size of a current maximumcoding unit, and the frequency transformer 430 has to determine a sizeof a transformation unit in each coding unit from among the coding unitshaving the tree structure.

FIG. 5 is a block diagram of an image decoder 500 based on a codingunit, according to an exemplary embodiment.

Referring to FIG. 5, a parser 510 parses encoded image data to bedecoded and information about encoding used for decoding from abitstream 505. The encoded image data is output as inverse quantizeddata through an entropy decoder 520 and an inverse quantizer 530, andthe inverse quantized data is restored to image data in a spatial domainthrough an inverse transformer 540.

An intra predictor 550 performs intra prediction on coding units in anintra mode with respect to the image data in the spatial domain, and amotion compensator 560 performs motion compensation on coding units inan inter mode by using a reference frame 585.

The image data in the spatial domain, which passed through the intrapredictor 550 and the motion compensator 560, may be output as arestored frame 595 after being post-processed through a deblocker 570and a loop filter 580. Also, the image data that is post-processedthrough the deblocker 570 and the loop filter 580 may be output as thereference frame 585.

In order to decode the image data in the image data decoder 230 of theapparatus 200, the image decoder 500 may perform operations that areperformed after the parser 510.

In order for the image decoder 500 to be applied to the apparatus 200,all elements of the image decoder 500, i.e., the parser 510, the entropydecoder 520, the inverse quantizer 530, the inverse transformer 540, theintra predictor 550, the motion compensator 560, the deblocker 570, andthe loop filter 580 have to perform operations based on coding unitshaving a tree structure for each maximum coding unit.

Specifically, the intra prediction 550 and the motion compensator 560have to determine partitions and a prediction mode for each of thecoding units having the tree structure, and the inverse transformer 540has to determine a size of a transformation unit for each coding unit.

FIG. 6 is a diagram illustrating partitions and deeper coding unitsaccording to depths, according to an exemplary embodiment.

The apparatus 100 and the apparatus 200 use hierarchical coding units soas to consider characteristics of an image. A maximum height, a maximumwidth, and a maximum depth of coding units may be adaptively determinedaccording to the characteristics of the image, or may be differently setby a user. Sizes of deeper coding units according to depths may bedetermined according to a preset maximum size of a coding unit.

In a hierarchical structure 600 of coding units, the maximum height andthe maximum width of the coding units are each 64, and the maximum depthis 4. Since a depth deepens along a vertical axis of the hierarchicalstructure 600, a height and a width of the deeper coding unit are eachsplit. Also, a prediction unit and a partition, which are a base forprediction encoding of each deeper coding unit, are shown along ahorizontal axis of the hierarchical structure 600.

In other words, a coding unit 610 is a maximum coding unit in thehierarchical structure 600, wherein a depth is 0 and a size, i.e., aheight by width, is 64×64. The depth deepens along the vertical axis,and a coding unit 620 having a size of 32×32 and a depth of 1, a codingunit 630 having a size of 16×16 and a depth of 2, a coding unit 640having a size of 8×8 and a depth of 3, and a coding unit 650 having asize of 4×4 and a depth of 4 exist. The coding unit 650 having the sizeof 4×4 and the depth of 4 is a minimum coding unit.

Partial data units are shown in FIG. 6 as the prediction units of acoding unit along the horizontal axis according to each depth. In otherwords, if the coding unit 610 having the size of 64×64 and the depth of0 is a prediction unit, the prediction unit may be split into partialdata units included in the encoding unit 610, i.e. a partial data unit610 having a size of 64×64, partial data units 612 having the size of64×32, partial data units 614 having the size of 32×64, or partial dataunits 616 having the size of 32×32.

A prediction unit of the coding unit 620 having the size of 32×32 andthe depth of 1 may be split into partial data units included in thecoding unit 620, i.e. a partial data unit 620 having a size of 32×32,partial data units 622 having a size of 32×16, partial data units 624having a size of 16×32, and partial data units 626 having a size of16×16.

A prediction unit of the coding unit 630 having the size of 16×16 andthe depth of 2 may be split into partial data units included in thecoding unit 630, i.e. a partial data unit having a size of 16×16included in the coding unit 630, partial data units 632 having a size of16×8, partial data units 634 having a size of 8×16, and partial dataunits 636 having a size of 8×8.

A prediction unit of the coding unit 640 having the size of 8×8 and thedepth of 3 may be split into partial data units included in the codingunit 640, i.e. a partial data unit having a size of 8×8 included in thecoding unit 640, partial data units 642 having a size of 8×4, partialdata units 644 having a size of 4×8, and partial data units 646 having asize of 4×4.

The coding unit 650 having the size of 4×4 and the depth of 4 is theminimum coding unit and a coding unit of the lowermost depth. Aprediction unit of the coding unit 650 is only assigned to a partialdata unit having a size of 4×4.

In order to determine a coded depth of the maximum coding unit 610, thecoding unit determiner 120 of the apparatus 100 has to perform encodingfor coding units corresponding to each depth included in the maximumcoding unit 610.

The number of deeper coding units according to depths including data inthe same range and the same size increases as the depth deepens. Forexample, four coding units corresponding to a depth of 2 are required tocover data that is included in one coding unit corresponding to a depthof 1. Accordingly, in order to compare encoding results of the same dataaccording to depths, the coding unit corresponding to the depth of 1 andfour coding units corresponding to the depth of 2 are each encoded.

In order to perform encoding according to depths, for a current depthfrom among the depths, a representative encoding error which is a leastencoding error may be selected for a current depth by performingencoding for prediction units in the coding units corresponding to thecurrent depth, along the horizontal axis of the hierarchical structure600. Alternatively, a minimum encoding error may be searched for bycomparing the least encoding errors according to depths, by performingencoding for each depth as the depth deepens along the vertical axis ofthe hierarchical structure 600. A depth and a partition having theminimum encoding error in the coding unit 610 may be selected as a codeddepth and a partition type of the coding unit 610.

FIG. 7 is a diagram for describing a relationship between a coding unit710 and transformation units 720, according to an exemplary embodiment.

The apparatus 100 or 200 encodes or decodes an image according to codingunits having sizes smaller than or equal to a maximum coding unit foreach maximum coding unit. Sizes of transformation units for frequencytransformation during encoding may be selected based on data units thatare not larger than a corresponding coding unit.

For example, in the apparatus 100 or 200, if a size of the coding unit710 is 64×64, frequency transformation may be performed by using thetransformation units 720 having a size of 32×32.

Also, data of the coding unit 710 having the size of 64×64 may beencoded by performing the frequency transformation on each of thetransformation units having the size of 32×32, 16×16, 8×8, and 4×4,which are smaller than 64×64, and then a transformation unit having aleast coding error may be selected.

FIG. 8 is a diagram for describing encoding information of coding unitsaccording to depths, according to an exemplary embodiment.

The output 130 of the apparatus 100 may encode and transmit information800 about a partition type, information 810 about a prediction mode, andinformation 820 about a size of a transformation unit for each codingunit corresponding to a coded depth, as information about an encodingmode.

The information 800 indicates information about a shape of a partitionobtained by splitting a prediction unit of a current coding unit, as adata unit for prediction encoding the current coding unit. For example,a current coding unit CU_0 having a depth 0 and size of 2N×2N may besplit into any one of a partition 802 having a size of 2N×2N, apartition 804 having a size of 2N×N, a partition 806 having a size ofN×2N, and a partition 808 having a size of N×N. Here, the information800 about a partition type of the current coding unit is set to indicateone of the partition 804 having a size of 2N×N, the partition 806 havinga size of N×2N, and the partition 808 having a size of N×N

The information 810 indicates a prediction mode of each partition. Forexample, the information 810 may indicate a mode of prediction encodingperformed on a partition indicated by the information 800, i.e., anintra mode 812, an inter mode 814, or a skip mode 816.

The information 820 indicates a transformation unit to be based on whenfrequency transformation is performed on a current coding unit. Forexample, the transformation unit may be a first intra transformationunit 822, a second intra transformation unit 824, a first intertransformation unit 826, or a second intra transformation unit 828.

The image data and encoding information extractor 220 of the apparatus200 may extract and use the information 800, 810, and 820 for decoding,according to each deeper coding unit.

FIG. 9 is a diagram illustrating deeper coding units according todepths, according to an exemplary embodiment.

Split information may be used to indicate a change in a depth. The spiltinformation indicates whether a coding unit of a current depth is splitinto coding units of a lower depth.

A prediction unit 910 for prediction encoding a coding unit having adepth of 0 and a size of 2N 0×2N 0 may include a partition type 912having a size of 2N 0×2N 0, a partition type 914 having a size of2N_0×N_0, a partition type 916 having a size of N_0×2N_0, and apartition type 918 having a size of N_0×N_0. Although the partitiontypes 912, 914, 916, and 918 in which prediction units are splitsymmetrically are shown in FIG. 9, partition types are not limitedthereto and may have asymmetrical, arbitrary, or geometric shapes asdescribed above.

Encoding via motion prediction is repeatedly performed on one partitionhaving a size of 2N_0×2N_0, two partitions having a size of 2N_0×N_0,two partitions having a size of N_0×2N_0, and four partitions having asize of N_0×N_0, according to each split type. The prediction encodingin an intra mode and an inter mode may be performed on the partitionshaving the sizes of 2N_0×2N_0, N_0×2N_0, 2N_0×N_0, and N_0×N_0. Theprediction encoding in a skip mode is performed only on the partitionhaving the size of 2N_0×2N_0.

If the encoding error is the smallest in any one of the partition types912, 914, and 916 having the sizes of 2N_0×2N_0, 2N_0×N_0, and N_0×2N_0,no more split to a lower depth is necessary.

If the encoding error is the smallest in the split type 918 having thesize N_0×N_0, a depth is changed from 0 to 1 to split the partition type918 in operation 920, and encoding is repeatedly performed on codingunits 930 having a depth of 2 and a size of N_0×N_0 to search for aminimum encoding error.

A prediction unit 940 for performing prediction encoding on the codingunit 930 having a depth of 1 and a size of 2N_1×2N_1 (=N_0×N_0) mayinclude a partition type 942 having a size of 2N_1×2N_1, a partitiontype 944 having a size of 2N_1×N_1, a partition type 946 having a sizeof N_1×2N_1, and a partition type 948 having a size of N_1×N_1.

Also, if the encoding error is the smallest in the partition type 948having the size N_1×N_1, the depth is changed from 1 to 2 to split thepartition type 948 in operation 950, and encoding is repeatedlyperformed on coding units 960 having a depth of 2 and a size of N_2×N_2to search for a minimum encoding error.

When a maximum depth is d, split information according to each depth maybe set up to when a depth becomes d−1, and split information may be setup to when a depth becomes d−2. That is, when encoding is performed froma depth of d−2 to a depth of d−1 in operation 970, a prediction unit 990for performing prediction encoding on a coding unit 980 having a depthof d−1 and a size of 2N_(d−1)×2N_(d−1) may include a partition type 992having a size of 2N_(d−1)×2N_(d−1), a partition type 994 having a sizeof 2N_(d−1)×N_(d−1), a partition type 996 having a size ofN_(d−1)×2N_(d−1), and a partition type 998 having a size ofN_(d−1)×N_(d−1).

Encoding via prediction encoding may be repeatedly performed on onepartition having a size of 2N_(d−1)×2N_(d−1), two partitions having asize of 2N_(d−1)×N_(d−1), two partitions having a size ofN_(d−1)×2N_(d−1), and four partitions having a size of N_(d−1)×N_(d−1),according to each split type to search for a partition type generating aminimum encoding error.

Although an encoding error of the partition type 998 having the size ofN_(d−1)×N_(d−1) is the smallest, since the maximum depth is d, a codingunit CU_(d−1) having a depth of d−1 is no longer split to a lower depth,and a coded depth of a current maximum coding unit 900 may be determinedto be a depth of d−1 and a partition type of the current maximum codingunit 900 may be determined to be N_(d−1)×N_(d−1). Also, since themaximum depth is d, split information about a coding unit 952 having adepth of d−1 is not set.

A data unit 999 may be referred to as a ‘minimum unit’ of a currentmaximum coding unit. The minimum unit may be a square data unit having asize obtained by splitting a minimum coding unit having a lowermostcoded depth by 4. By repeatedly performing encoding, the apparatus 100may compare encoding errors according to depths of the coding unit 900,select a depth that generates a least encoding error, determine a codeddepth, and set a corresponding partition type and prediction mode as anencoding mode of the coded depth.

As such, minimum encoding errors according to depths of depths may becompared in all of the depths of 0, 1, . . . , d−1, and d, and a depthhaving a smallest error may be selected as a coded depth. A coded depth,and a prediction mode and a partition type of a prediction unit may beencoded and transmitted as information about an encoding mode. Also,since a coding unit has to be split up to when a depth changes from adepth of 0 to a coded depth, only split information of the coded depthhas to be set to ‘0’ and split information according to each depthexcluding the coded depth has to be set to ‘1’.

The image data and encoding information extractor 220 of the apparatus200 may extract and use information about a prediction unit and a codeddepth of the coding unit 900 to decode the partition type 912. Theapparatus 200 may determine a depth in which split information is ‘0’ asa coded depth by using the split information according to each depth,and may use information about an encoding mode of the correspondingdepth for decoding.

FIGS. 10, 11, and 12 are diagrams for describing a relationship betweencoding units 1010, prediction units 1060, and frequency transformationunits 1070, according to an exemplary embodiment.

The coding units 1010 are coding units corresponding to coded depthsdetermined by the apparatus 100, in a maximum coding unit 1000. Theprediction units 1060 are partitions of prediction units of each of thecoding units 1010, and the transformation units 1070 are transformationunits of each of the coding units 1010.

When a depth of a maximum coding unit is 0 in the coding units 1010,depths of coding units 1012 and 1054 are 1, depths of coding units 1014,1016, 1018, 1028, 1050, and 1052 are 2, depths of coding units 1020,1022, 1024, 1026, 1030, 1032, and 1048 are 3, and depths of coding units1040, 1042, 1044, and 1046 are 4.

In the prediction units 1060, some partitions 1014, 1016, 1022, 1032,1048, 1050, 1052, and 1054 are obtained by splitting the coding units inthe encoding units 1010. In other words, partition types in thepartitions 1014, 1022, 1050, and 1054 have a size of 2N×N, partitiontypes in the partitions 1016, 1048, and 1052 have a size of N×2N, and apartition type of the partition 1032 has a size of N×N. Prediction unitsand partitions of the coding units 1010 are smaller than or equal toeach coding unit.

Frequency transformation or frequency inverse transformation isperformed on image data of the coding unit 1052 in the transformationunits 1070 in a data unit that is smaller than the coding unit 1052.Also, the transformation units 1014, 1016, 1022, 1032, 1048, 1050, and1052 in the transformation units 1070 are different from those in theprediction units 1060 in terms of sizes and shapes. In other words, theapparatuses 100 and 200 may perform intra prediction, motion estimation,motion compensation, frequency transformation, and inversetransformation individually on a data unit in the same coding unit.

Accordingly, since an optimum coding unit is determined by recursivelyperforming encoding on coding units having a hierarchical structure ineach region for each maximum coding unit, coding units having arecursive tree structure may be obtained. Encoding information mayinclude split information about a coding unit, information about apartition type, information about a prediction mode, and informationabout a size of a transformation unit. Table 1 shows encodinginformation which may be set by the apparatus 100 and the apparatus 200.

TABLE 1 Split information 0 (encoding of coding Split information unithaving current depth of d and size of 2N × 2N) 1 Prediction Partitiontype Size of transformation unit mode intra Asymmetric symmetric Splitinformation Split information inter partition partition 0 oftransformation 1 of transformation type type unit unit skip (only 2N ×2N 2N × nU 2N × 2N N × N (symmetric Repeatedly encode 2N × 2N) 2N × N 2N × nD partition type) coding units  N × 2N nL × 2N N/2 × N/2 havinglower N × N nR × 2N (asymmetric depth of d + 1 partition type)

The output unit 130 of the apparatus 100 may output encoding informationabout coding units having a tree structure, and the image data andencoding information extractor 220 of the apparatus 200 may extract theencoding information about the coding units having the tree structurefrom a received bitstream.

Split information indicates whether a current coding unit is split intocoding units of a lower depth. When split information of a current depthof d is 0, since a depth in which the current coding unit is no longersplit into a lower coding unit is a coded depth, information about apartition type, information about a prediction mode, and informationabout a size of a transformation unit may be defined for the codeddepth. When the current coding unit needs to be further split accordingto the split information, encoding has to be independently performed on4 split coding units of a lower depth.

A prediction mode may be expressed as one of an intra mode, an intermode, and a skip mode. The intra mode and the inter mode may be definedin all partition types, and the skip mode may be defined only in apartition type having a size of 2N×2N.

Information about a partition type may indicate symmetric partitiontypes having sizes of 2N×2N, 2N×N, N×2N, and N×N obtained bysymmetrically splitting a height or a width of a prediction unit, andasymmetric partition types having sizes of 2N×nU, 2N×nD, nL×2N, andnR×2N obtained by asymmetrically splitting the height or the width ofthe prediction unit. The asymmetric partition types having the sizes of2N×nU and 2N×nD are respectively obtained by splitting the height in 1:3and 3:1, and the asymmetric partition types having the sizes of nL×2Nand nR×2N are obtained by splitting the width in 1:3 and 3:1.

A size of a transformation unit may be set to be two types in an intramode, and two types in an inter mode. That is, when split informationabout the transformation unit is 0, a size of the transformation unit isset to 2N×2N which is a size of a current coding unit. When splitinformation of the transformation unit is 1, the transformation unit maybe obtained by splitting the current coding unit. Also, when a partitiontype having a size obtained by splitting the current coding unit is anasymmetric partition type, a size of the transformation unit may be setto N×N, and when a partition type having a size obtained by splittingthe current coding unit is a symmetric partition type, a size of thetransformation unit may be set to N/2×N/2.

Encoding information of coding units having a tree structure may includeat least one of a coding unit of a coded depth, a prediction unit, and aminimum unit. The coding unit of the coded depth may include at leastone of a prediction unit and a minimum unit containing the same encodinginformation.

Accordingly, it is determined whether adjacent data units are includedin the same coding unit corresponding to the coded depth by comparingencoding information of the adjacent data units. Also, since acorresponding coding unit of the coded depth is determined by usingencoding information of a data unit, a distribution of coded depths in amaximum coding unit may be determined.

Accordingly, in this case, when a current coding unit is predicted basedon neighboring data units, encoding information of data units in deepercoding units adjacent to the current coding unit may be directlyreferred to and used.

Alternatively, when a current coding unit is predicted based onneighboring coding units, data units adjacent to the current coding unitare searched by using encoded information of the data units, and thesearched adjacent coding units may be referred for predicting thecurrent coding unit.

FIG. 13 is a diagram illustrating a relationship between a coding unit,a prediction unit, and a transformation unit according to encoding modeinformation of Table 1, according to an exemplary embodiment.

A maximum coding unit 1300 includes coding units 1302, 1304, 1306, 1312,1314, 1316, and 1318. Since the coding unit 1318 is a coding unit of acoded depth, split information may be set to 0. Partition typeinformation of the coding unit 1318 having a size of 2N×2N may be set toone of a partition type 1322 having a size of 2N×2N, a partition type1324 having a size of 2N×N, a partition type 1326 having a size of N×2N,a partition type 1328 having a size of N×N, a partition type 1332 havinga size of 2N×nU, a partition type 1334 having a size of 2N×nD, apartition type 1336 having a size of nL×2N, and a partition type 1338having a size of nR×2N.

Assuming that partition type information is set to one of the partitiontypes 1322 having the size of 2N×2N, the partition type 1324 having thesize of 2N×N, the partition type 1326 having the size of N×2N, and thepartition type 1328 having the size of N×N, which are symmetricpartition types, when split information (TU size flag) of atransformation unit is 0, a transformation unit 1342 having a size of2N×2N may be set, and when split information of a transformation unit is1, a transformation unit 1344 having a size of N×N may be set.

Assuming that partition type information is set to one of the partitiontypes 1332 having the size of 2N×nU, the partition type 1334 having thesize of 2N×nD, nL×2N(1336), and the partition type 1338 having the sizeof nR×2N, when split information (TU size flag) of a transformation unitis 0, a transformation unit 1352 having a size of 2N×2N may be set, andwhen transformation unit split information is 1, a transformation unit1354 having a size of N/2×N/2 may be set.

Intra prediction performed on a prediction unit by the intra predictor410 of the apparatus 100 of FIG. 4 and the intra predictor 550 of theapparatus 200 of FIG. 5 will be explained in detail.

FIGS. 14A through 14C are diagrams illustrating formats of a luminancecomponent image and a chrominance component image, according to theexemplary embodiments.

Each coding unit forming one frame may be expressed by using one ofthree components, i.e., Y, Cb, and Cr. Y is luminance data havingluminance information, and Cb and Cr are chrominance data havingchrominance information.

The chrominance data may be expressed using a lower amount of data thanthe luminance data, based on a fact that a person is generally moresensitive to the luminance information than the chrominance information.Referring to FIG. 14A, one coding unit having a 4:2:0 format includesluminance data 1410 having a size of H×W (H and W are positiveintegers), and two pieces of chrominance data 1420 and 1430 having asize of (H/2)×(W/2) obtained by sampling the chrominance components Cband Cr by ¼. Referring to FIG. 14B, one coding unit having a 4:2:2format includes luminance data 1440 having a size of H×W (H and W arepositive integers), and two pieces of chrominance data 1450 and 1460having a size of H×(W/2) obtained by sampling the chrominance componentsCb and Cr by ½ in a horizontal direction. Also, referring to FIG. 14C,when one coding unit has a 4:4:4 format, the coding unit includesluminance data 1470, and chrominance data 1480 and 1490 each having asize of H×W without sampling the chrominance components Cb and Cr, so asto precisely express a chrominance component image.

Hereinafter, it is assumed that the luminance component coding unit andthe chrominance component coding unit, which are intra predicted, areone of image signals having color formats of 4:2:0, 4:2:2, and 4:4:4defined in a YCbCr (or YUV) color domain. It would be understood by oneof ordinary skill in the art that the embodiments may be applied to notonly an image including a luminance component and a chrominance imagebut also an image including a plurality of different image components.

FIG. 15 is a table showing the number of intra prediction modesaccording to sizes of luminance component coding units, according to anexemplary embodiment.

The number of intra prediction modes to be applied to a luminancecomponent coding unit (a decoding unit in a decoding process) may bevariously set. For example, referring to FIG. 15, if the size of aluminance component coding unit on which intra prediction is performedis N×N, the numbers of intra prediction modes actually performed on 2×2,4×4, 8×8, 16×16, 32×32, 64×64, and 128×128-sized luminance componentcoding units may be respectively set as 5, 9, 9, 17, 33, 5, and 5 (inExample 2). The numbers of intra prediction modes to be actuallyperformed are differently set according to the sizes of luminancecomponent coding units because overheads for encoding prediction modeinformation differ according to the sizes of the luminance componentcoding units. In other words, a small luminance component coding unitoccupies a small portion of entire image data but may have a largeoverhead in order to transmit additional information such as predictionmode information of the luminance component coding unit. Accordingly, ifa luminance component small coding unit is encoded by using anexcessively large number of prediction modes, the number of bits may beincreased and thus compression efficiency may be reduced. Also, aluminance component large coding unit, e.g., a luminance componentcoding unit equal to or greater than 64×64, generally corresponds to aplane region of image data, and thus encoding of the large luminancecomponent coding unit by using an excessively large number of predictionmodes may also reduce compression efficiency. Accordingly, a size of aprediction unit is too greater or less than a predetermined size, only asmaller number of intra prediction modes may be used. The number ofintra prediction modes used according to a size of a prediction unit maybe variously set without being limited to FIG. 15. The number ofprediction units used according to a size of each prediction unit ofFIG. 15 is exemplary, and the number of prediction units according to asize of each prediction unit may be changed.

FIG. 16A is a table showing intra prediction modes applied to aluminance component coding unit having a predetermined size, accordingto an exemplary embodiment.

Referring to FIGS. 15 and 16A, for example, when intra prediction isperformed on a luminance component coding unit having a 4×4 size, theluminance component coding unit may have a vertical mode (mode 0), ahorizontal mode (mode 1), a direct current (DC) mode (mode 2), adiagonal down-left mode (mode 3), a diagonal down-right mode (mode 4), avertical-right mode (mode 5), a horizontal-down mode (mode 6), avertical-left mode (mode 7), and a horizontal-up mode (mode 8).

FIG. 16B illustrates directions of the intra prediction modes shown inFIG. 16A. In FIG. 16B, numbers at the ends of the arrows representprediction modes corresponding to prediction directions indicated by thearrows. Here, mode 2 is a DC mode having no directionality and thus isnot shown in FIG. 16B.

FIG. 16C is a diagram for describing a method of performing intraprediction on a luminance component coding unit by using the intraprediction modes shown in FIG. 16A, according to an exemplaryembodiment.

Referring to FIG. 16C, a prediction coding unit is generated accordingto an available intra prediction mode determined according to the sizeof a current luminance component coding unit by using neighboring pixelsA through M of the current luminance component coding unit. For example,an operation of performing prediction encoding on a current coding unithaving a 4×4 size according to mode 0, i.e., a vertical mode, shown inFIG. 16A will be described. Initially, values of the neighboring pixelsA through D at an upper side of the current coding unit are predicted aspixel values of the current coding unit. That is, the value of theneighboring pixel A is predicted as a value of four pixels in a firstcolumn of the current coding unit, the value of the neighboring pixel Bis predicted as a value of four pixels in a second column of the currentcoding unit, the value of the neighboring pixel C is predicted as avalue of four pixels in a third column of the current coding unit, andthe value of the neighboring pixel D is predicted as a value of fourpixels in a fourth column of the current coding unit. After that, thepixel values of the current coding unit predicted by using theneighboring pixels A through D are subtracted from the pixel values ofthe original current coding unit so as to calculate an error value andthen the error value is encoded.

FIG. 17 is a diagram for describing intra prediction modes applied to aluminance component coding unit having a predetermined size, accordingto another exemplary embodiment.

Referring to FIGS. 15 and 17, for example, when intra prediction isperformed on a coding unit having a 2×2 size, the coding unit may have atotal of five modes such as a vertical mode, a horizontal mode, a DCmode, a plane mode, and a diagonal down-right mode.

An intra prediction mode of a luminance component prediction unit mayinclude intra prediction modes that determine a neighboring referencepixel by using a line having a predetermined gradient with respect to apixel in a prediction unit and uses the determined neighboring referencepixel. The gradient may be set by using (dx, dy) parameters (dx and dyare integers). For example, if each of the 33 prediction modes isdefined as mode N (where N is an integer from 0 to 32), mode 0 may beset as a vertical mode, mode 1 may be set as a horizontal mode, mode 2may be set as a DC mode, mode 3 may be set as a plane mode, and each ofmode 4 through mode 31 may be defined as a prediction mode thatdetermines a neighboring reference pixel by using a line having adirectionality of tan−1(dy/dx) by using (dx, dy) represented as one of(1,−1), (1,1), (1,2), (2,1), (1,−2), (2,1), (1,−2), (2,−1), (2,−11),(5,−7), (10,−7), (11,3), (4,3), (1,11), (1,−1), (12,−3), (1,−11),(1,−7), (3,−10), (5,−6), (7,−6), (7,−4), (11,1), (6,1), (8,3), (5,3),(5,7), (2,7), (5,−7), and (4,−3) as shown in Table 2 and uses thedetermined neighboring reference pixel for prediction.

TABLE 2 mode # dx dy mode 4 1 −1 mode 5 1 1 mode 6 1 2 mode 7 2 1 mode 81 −2 mode 9 2 −1 mode 10 2 −11 mode 11 5 −7 mode 12 10 −7 mode 13 11 3mode 14 4 3 mode 15 1 11 mode 16 1 −1 mode 17 12 −3 mode 18 1 −11 mode19 1 −7 mode 20 3 −10 mode 21 5 −6 mode 22 7 −6 mode 23 7 −4 mode 24 111 mode 25 6 1 mode 26 8 3 mode 27 5 3 mode 28 5 7 mode 29 2 7 mode 30 5−7 mode 31 4 −3 Mode 0 is a vertical mode, mode 1 is a horizontal mode,mode 2 is a DC mode, mode 3 is a plane mode, and mode 32 is a bi-linearmode.

Mode 32 may be set as a bi-linear mode that uses bi-linear interpolationas will be described later with reference to FIG. 19.

FIGS. 18A through 18C are reference diagrams for describing intraprediction modes having various directionalities, according to anexemplary embodiment.

As described above with reference to Table 2, in each of the intraprediction modes, a neighboring reference pixel may be determined byusing a line having a gradient of tan⁻¹(dy/dx) determined by using aplurality of (dx, dy) parameters, and prediction may be performed byusing the determined neighboring reference pixel.

Referring to FIG. 18A, neighboring pixels A and B on a line 180 thatextends from a current pixel P in a current luminance component codingunit, which is to be predicted, at an angle of tan⁻¹(dy/dx) determinedby a value of a (dx, dy) parameter according to a mode, as shown inTable 1, may be used as predictors of the current pixel P. In this case,the neighboring pixels A and B may be pixels that have been encoded andrestored, and belong to previous coding units located above and to theleft side of the current coding unit. Also, when the line 180 does notpass through a neighboring pixels on locations each having an integralvalue but passes between these neighboring pixels, a neighboring pixelcloser to the line 180 may be used as a predictor of the current pixelP. Alternatively, a weighted average value considering a distancebetween intersections of the line 180 and neighboring pixels close tothe line 180 may be used as a predictor of the current pixel P.

FIGS. 18B and 18C are reference diagrams for describing a process ofgenerating a predictor when the line 180 of FIG. 18A passes betweeninteger location neighboring pixels on locations each having an integralvalue, not passes through neighboring pixels on locations each having anintegral value, according to an exemplary embodiment.

Referring to FIG. 18B, when the line 180 having an angle of tan⁻¹(dy/dx)determined by values of (dx, dy) parameters according to modes passesbetween neighboring pixels A 181 and B 182 on locations having integralvalues, a weighted average value considering a distance betweenintersections of the line 180 and the neighboring pixels A 181 and B 182close the line 180 may be used as a predictor of the current pixel P.For example, when a distance between the neighboring pixel A 181 and theintersection of the line 180 having the angle of tan⁻¹(dy/dx) is f and adistance between the neighboring pixel B 182 and the intersection is g,a predictor of the current pixel P may be obtained as (A*g+B*f)/(f+g).The distances f and g may be distances normalized with integers. Whensoftware or hardware is embodied, a predictor of the current pixel P maybe obtained through a shift operation as (g*A+f*B+2)>>2. As shown inFIG. 18B, when the line 180 passes a location corresponding to ¼ closeto the neighboring pixel A 181 from among points obtained by splitting aspace between the neighboring pixel A 181 and the neighboring pixel B182 on locations having integral values by 4, a predictor of the currentpixel P may be obtained as (3*A+B)/4. Such calculation may be performedthrough shift operation considering rounding as in (3*A+B+2)>>2.

When the line 180 having the angle of tan⁻¹(dy/dx) determined by thevalues of the (dx, dy) parameters according to the modes passes betweenthe neighboring pixels A 181 and B 182 on locations having integralvalues, a space between the neighboring pixel A 181 and the neighboringpixel B 182 may be split into a predetermined number of regions, and aweighted average value considering a distance between intersections andthe neighboring pixel A 181 and the neighboring pixel B 182 according tothe split regions may be used as a prediction value. For example,referring to FIG. 18C, a space between the neighboring pixel A 181 andthe neighboring pixel B 182 may be split into 5 regions (P1 through P5),a representative weighted average value considering a distance betweenintersections and the neighboring pixel A 181 and the neighboring pixelB 182 according to the regions may be determined, and the representativeweighted average value may be used as a predictor of the current pixelP. In detail, when the line 180 passes the region P1, a value of theneighboring pixel A may be determined as a predictor of the currentpixel P. When the line 180 passes the region P2, (3*A+1*B+2)>>2 which isa weighted average value considering a distance between a midpoint ofthe region P2 and the neighboring pixels A and B may be determined as apredictor of the current pixel P. When the line 180 passes the regionP3, (2*A+2*B+2)>>2 which is a weighted average value considering adistance between a midpoint of the region P3 and the neighboring pixelsA and B may be determined as a predictor of the current pixel P. Whenthe line 180 passes through the region P4, (1*A+3*B+2)>>2 which is aweighted average value considering a distance between a midpoint of theregion P4 and the neighboring pixels A and B may be determined as apredictor of the current pixel P. When the line passes through theregion P5, a value of the neighboring pixel B may be determined as apredictor of the current pixel P.

Also, when two neighboring pixels, that is, the neighboring pixel Adisposed over the line 180 and the neighboring pixel B disposed at theleft of the line 180, meet the line 180, an average value of theneighboring pixel A and the neighboring pixel B may be used as apredictor of the current pixel. Alternatively, when a value of dx*dy isa positive value, the neighboring pixel A may be used, and when a valueof dx*dy is a negative value, the neighboring pixel B may be used.

The intra prediction modes having various directionalities shown inTable 2 may be preset by an encoding side and a decoding side, and onlyan index of an intra prediction mode of each coding unit may betransmitted.

FIG. 19 is a reference diagram for describing a bi-linear mode accordingto an exemplary embodiment.

Referring to FIG. 19, in the bi-linear mode, a geometric average iscalculated by considering a value of a current pixel P in a currentluminance component coding unit, which is to be predicted, values ofpixels on upper, lower, left, and right boundaries of the currentluminance component coding unit, and the distances between the currentpixel P and the upper, lower, left, and right boundaries of the currentluminance component coding unit, and the geometric average is then usedas a predictor of the current pixel P. For example, in the bi-linearmode, a geometric average calculated using a virtual pixel A 161, avirtual pixel B 162, a pixel D 166, and a pixel E 167 located to theupper, lower, left, and right sides of the current pixel P, and thedistances between the current pixel P and the upper, lower, left, andright boundaries of the current luminance component coding unit, is usedas a predictor of the current pixel P. Since the bi-linear mode is oneof the intra prediction modes, upper and left neighboring pixels thathave been encoded and restored have to be used as reference pixels forprediction. Thus, pixel values in the current luminance component codingunit are not used but virtual pixel values calculated using neighboringpixels located to the upper and left sides of the current luminancecomponent coding unit are used as the pixel A 161 and the pixel B 162.Specifically, first, a value of a virtual pixel C 163 on a lowerrightmost point of the current luminance component coding unit iscalculated by calculating an average of values of a neighboring pixel(right-up pixel) 164 on an upper rightmost point of the currentluminance component coding unit and a neighboring pixel (left-downpixel) 165 on a lower leftmost point of the current luminance componentcoding unit, as expressed in Equation 1 below:C=0.5(DownPixel+UpPixel)  [Equation 1]

Next, a value of the virtual pixel A 161 located on a lowermost boundaryof the current luminance component coding unit when the current pixel Pis extended downward by considering the distance W1 between the currentpixel P and the left boundary of the current luminance component codingunit and the distance W2 between the current pixel P and the rightboundary of the current luminance component coding unit, is calculatedby using Equation 2 below:A=(C*W1+LeftDownPixel*W2)/(W1+W2)  [Equation 2]

Similarly, a value of the virtual pixel B 162 located on a rightmostboundary of the current luminance component coding unit when the currentpixel P is extended in the right direction by considering the distanceh1 between the current pixel P and the upper boundary of the currentluminance component coding unit and the distance h2 between the currentpixel P and the lower boundary of the current luminance component codingunit, is calculated by using Equation 3 below:B=(C*h1+RightUpPixel*h2)/(h1+h2)  [Equation 3]

When the values of the virtual pixels A 161 and B 162 are determinedusing Equations 1 to 3, an average of the values of the pixels A 161,the pixel B, the pixel D 166, and the pixel E 167 may be used as apredictor of the current pixel P. In detail, an average value of thepixels A+B+C+D may be used as a predictor of the current pixel P, or aweighted average value considering a distance between the current pixelP and the virtual pixels A 161, B 162, D 166, and E 167 may be used as apredictor of the current picture P. For example, when a weighted averagevalue is used, a predictor of the current pixel P may be obtained as{(h1*A+h2*D)/(h1+h2)+(W1*B+W2*E)/(W1+W2)}/2. Such a bi-linear predictionprocess may be applied to all pixels in a current prediction unit, and aprediction value of the current prediction unit according to thebi-linear prediction mode may be generated.

According to the present embodiment, since prediction encoding isperformed according to intra prediction modes variously set according toa size of a prediction unit, more efficient compression may be achievedaccording to characteristics of an image.

As described above with reference to FIGS. 18A through 18C, when apredictor of the current pixel P is generated by using a neighboringpixel on the line 180 or close to the line 180, the line 180 hasactually a directionality of tan⁻¹(dy/dx). Since subtraction (dy/dx) isused to determine a neighboring pixel using the line 180, calculationusing a decimal point may be included when hardware or software isembodied, thereby increasing the amount of calculation. Accordingly,when a prediction direction for selecting a neighboring pixel to be usedas a reference pixel is set by using dx and dy parameters, like in Table2, dx and dy need to be set to reduce the amount of calculation.

FIG. 20 is a graph for describing a relationship between a current pixeland a neighboring pixel located on a line having a directionality of(dx, dy), according to an exemplary embodiment.

Referring to FIG. 20, a current pixel located on (j, i) is P 2010, andan upper neighboring pixel and a left neighboring pixel which passthrough the current pixel P 2010 and are located on a line having adirectionality, that is, a gradient, of tan⁻¹(dy/dx) are respectively A2011 and B 2012. It is assumed that locations of upper neighboringpixels correspond to an X axis on a coordinate plane and locations ofleft neighboring pixels correspond to a y axis on the coordinate plane.Also, it is assumed that a size of a prediction unit included in thecurrent pixel P 2010 is nS×nS (nS is a positive integer), a location ofeach of pixels of the prediction unit is one from (0,0) to (nS−1, nS−1),a location of an upper neighboring pixel located on the x axis is (m,−1) (m is an integer), and a location of a left neighboring pixellocated on the y axis is (−1, n) (n is an integer). A location of theupper neighboring pixel A 2011 meeting the line passing through thecurrent pixel P 2010 is (j+i*dx/dy, −1), and a location of the leftneighboring pixel B 2012 meeting the line is (−1, i+j*dy/dx).Accordingly, in order to determine the upper neighboring pixel A 2011 orthe left neighboring pixel B 2012 to predict the current pixel P 2010,subtraction such as dx/dy or dy/dx is necessary. Since subtraction hashigh complexity, a calculation speed may be reduced when software orhardware is embodied as described above. Accordingly, alternatively, avalue of at least one of dx and dy representing a directionality of aprediction mode for determining a neighboring pixel may be determined tobe an exponential power of 2. That is, when n and m are integers, dx anddy may be respectively 2{circumflex over ( )}n and 2{circumflex over( )}m.

Referring to FIG. 20, when the left neighboring pixel B 2012 is used asa predictor of the current pixel P 2010 and dx has a value of2{circumflex over ( )}n, calculation j*dy/dx necessary to determine (−1,i+j*dy/dx) which is the location of the left neighboring pixel B 2012 is(i*dy)/(2{circumflex over ( )}n), and calculation including subtractionusing an exponential power of 2 may be obtained through shift operationas (i*dy)>>n, thereby reducing the amount of calculations.

Likewise, when the upper neighboring pixel A 2011 is used as a predictorof the current pixel P 2010 and dy has a value of 2{circumflex over( )}m, calculation i*dx/dy necessary to determine (j+i*dx/dy,−1) whichis the location of the upper neighboring pixel A may be(i*dx)/(2{circumflex over ( )}m), and calculation including subtractionusing an exponential power of 2 may be obtained as (i*dx)>>m throughshift operation.

FIG. 21 is a graph for describing a change in a neighboring pixellocated on a line having a directionality of (dx, dy) according to alocation of a current pixel, according to an exemplary embodiment.

Referring to FIG. 21, a location of a neighboring pixel referred toaccording to an intra prediction mode direction is changed according towhether a current pixel to be predicted is a pixel P 2110 or a pixel Q2120. For example, when the pixel P 2110 is predicted, an upper pixel Ais used, and when the pixel Q 2120 is predicted, a left pixel B is used.When only a dy component in a y direction from among (dx, dy) indicatinga prediction direction has a value corresponding to an exponential powerof 2 as 2{circumflex over ( )}m, a location of the upper pixel A 2111 inFIG. 21 may be determined through shift operation without subtraction as(j+(i*dx)>>m, −1) and a location of the left pixel B 2121 does not needsubtraction as (−1, a+b*2{circumflex over ( )}m/dx). Accordingly, inorder to exclude subtraction when predictors of all pixels of a currentblock are generated, all of dx and dy may have exponential powers of 2.

FIGS. 22 and 23 are diagrams for describing a method of determining anintra prediction mode direction, according to an exemplary embodiment.

In general, linear patterns in an image or video signal are vertical orhorizontal in many cases. Accordingly, when an intra prediction modehaving various directionalities is defined by using (dx, dy) parameters,image encoding efficiency may be improved by defining values of dx anddy as follows.

In detail, when dy has a fixed value of 2{circumflex over ( )}m, anabsolute value of dx may be set such that an interval between predictiondirections close to a vertical direction is narrow and an intervalbetween prediction directions close to a horizontal direction is wide.For example, referring to FIG. 22, when dy is 2{circumflex over ( )}5,that is, has a value of 32, a value of dx may be set to 2, 5, 9, 13, 17,21, 26, 32, −2, −5, −9, −13, −17, −21, −26, and −32 such that aninterval between prediction directions close to a vertical direction isnarrow and an interval between prediction directions close to ahorizontal direction is wide.

Likewise, when dx has a fixed value of 2{circumflex over ( )}n, anabsolute value of dy may be set such that an interval between predictiondirections close to a horizontal direction is narrow and an intervalbetween prediction directions close to a vertical direction is wide. Forexample, referring to FIG. 23, when dx is 2{circumflex over ( )}5, thatis, has a value of 32, a value of dy may be set to 2, 5, 9, 13, 17, 21,26, 32, −2, −5, −9, −13, −17, −21, −26, and −32 such that an intervalbetween prediction directions close to a horizontal direction is narrowand an interval between prediction directions close to a verticaldirection is wide.

When a value of any one of dx and dy is fixed, a non-fixed remainingvalue may be set to increase according to prediction modes. For example,when dy is fixed, an interval between values of dx may be set toincrease by a predetermined value. Also, an angle between a horizontaldirection and a vertical direction may be divided according topredetermined units, and an increased amount may be set according to thedivided angles. For example, when dy is fixed, a value of dx may be setto have an increased width of a when an angle with respect to a verticalaxis is less than 15°, an increased width of b when an angle ranges from15 to 30°, and an increased width of c when an angle is equal to orgreater than 30°.

For example, prediction modes having a directionality of tan−1(dy/dx)using (dx, dy) may define (dx, dy) parameters shown in Tables 3 through5.

TABLE 3 dx Dy dx dy dx dy −32 32 21 32 32 13 −26 32 26 32 32 17 −21 3232 32 32 21 −17 32 32 −26 32 26 −13 32 32 −21 32 32 −9 32 32 −17 −5 3232 −13 −2 32 32 −9 0 32 32 −5 2 32 32 −2 5 32 32 0 9 32 32 2 13 32 32 517 32 32 9

TABLE 4 dx Dy dx dy dx dy −32 32 19 32 32 10 −25 32 25 32 32 14 −19 3232 32 32 19 −14 32 32 −25 32 25 −10 32 32 −19 32 32 −6 32 32 −14 −3 3232 −10 −1 32 32 −6 0 32 32 −3 1 32 32 −1 3 32 32 0 6 32 32 1 10 32 32 314 32 32 6

TABLE 5 dx Dy dx dy dx dy −32 32 23 32 32 15 −27 32 27 32 32 19 −23 3232 32 32 23 −19 32 32 −27 32 27 −15 32 32 −23 32 32 −11 32 32 −19 −7 3232 −15 −3 32 32 −11 0 32 32 −7 3 32 32 −3 7 32 32 0 11 32 32 3 15 32 327 19 32 32 11

As described above, each of intra prediction modes using (dx, dy)parameters uses a left neighboring pixel having a location (−1,i+j*dy/dx) or an upper neighboring pixel having a location(j+i*dx/dy,−1) as a predictor of a pixel located on (j, i). When atleast one of values of dx and dy has an exponential power of 2 as shownin Table 3, the location (−1, i+j*dy/dx) of the left neighboring pixeland the location (j+i*dx/dy,−1) of the upper neighboring pixel may beobtained by using only multiplication and shift operations withoutsubtraction. Since subtraction using dx having a value of 2{circumflexover ( )}n as shown in a case where dx from among values of (dx, dy) is32 may be replaced with right shift operation, the location of the leftneighboring pixel may be obtained without subtraction based on a valueof (i*dy)>>n. Likewise, since subtraction using dy having a value of2{circumflex over ( )}m as shown in a case where dy from among values of(dx, dy) is 32 as shown in Table 3 may be replaced with a right shiftoperation, the location of the right neighboring pixel may be obtainedwithout subtraction based on a value of (i*dx)>>m.

FIG. 24 is a reference diagram for describing an available intraprediction mode according to an exemplary embodiment.

Referring to FIG. 24, in order to obtain a predictor of a current pixel,a predictor P1 obtained through horizontal linear interpolation of thecurrent pixel and a predictor P2 obtained through vertical linearinterpolation of the current pixel may be generated and an average valueof the predictors P1 and P2 may be used as a predictor of the currentpixel. An intra prediction mode of FIG. 24 is defined as a planar mode.According to the planar mode, pixels on a rightmost column are generatedby copying a rightmost pixel T 244 from among pixels of the sameprediction unit which are located on an uppermost column of theprediction unit to pixels of the rightmost column. A predictor P1 243 isobtained through horizontal linear interpolation using pixels of aleftmost column and a rightmost column which are located on the same rowas the current pixel. As shown in FIG. 24, the predictor P1 243 throughthe horizontal linear interpolation of the current pixel is obtained byusing linear interpolation, for example, a geometric average valueconsidering a distance or an average value, of the generated pixel T ofthe rightmost column by copying a pixel PL 242 and the pixel 244 in thesame prediction unit of the leftmost column which are located on thesame location as the current pixel.

Likewise, according to the planar mode, pixels of a lowermost column aregenerated by copying the pixel L 248 which is located on a lowermostside from among pixels of the same prediction unit which are located ona leftmost column of a prediction unit to the pixels of the lowermostcolumn. The predictor P 246 is obtained through vertical linearinterpolation using pixels of an uppermost row and a lowermost row whichare located on the same column as the current pixel. As shown in FIG.24, the predictor P2 246 through vertical linear interpolation of thecurrent pixel is obtained through linear interpolation of the generatedpixel L of the lowermost row by copying a pixel PT 245 and the pixel L248 in the same prediction unit of the uppermost row which are locatedon the same column as the current pixel. In the planar mode, an averagevalue of the horizontal predictor P 243 and the vertical predictor P2246, that is, P1+P2>>1, is determined as a final predictor of thecurrent pixel.

FIGS. 25A and 25B are reference diagrams for describing an intraprediction mode according to another exemplary embodiment.

There is a predetermined correlation, for example, a linear correlation,between corresponding luminance pixels and chrominance pixels. Referringto FIGS. 25A and 25B, when a pixel at a location (x, y) of a luminanceblock 2520 which is previously restored is Rec_L′(x,y), a predictionvalue Pred_c(x,y) at the location (x, y) of a chrominance block 2510 maybe obtained by using a linear relationship between a luminance signaland a chrominance signal by using Equation 4.Pred_c(x,y)=a·Rec_L′(x,y)+b  [Equation 4]

In Equation 4, “a” is a parameter indicating a weight and “b” is aparameter indicating an offset. Values of the parameters “a” and “b” maybe obtained through calculation using neighboring pixels 2511 and 2522Rec_L′(i) of the luminance block 2520 and neighboring pixels 2511 and2512 Rec_c(i) (i=0, . . . , 2nS−1) of the chrominance block 2510 asshown in Equations 5 and 6.

                                     [Equation  5]$a = {\frac{{I \cdot {\sum\limits_{i = 0}^{I - 1}{{Rec\_ c}{(i) \cdot {Rec\_ L}^{\prime}}(i)}}} - {\sum\limits_{i = 0}^{I - 1}{{Rec\_ c}{(i) \cdot {\sum\limits_{i = 0}^{I - 1}{{Rec\_ L}^{\prime}(i)}}}}}}{{I \cdot {\sum\limits_{i = 0}^{I - 1}{{Rec\_ L}^{\prime}{(i) \cdot {Rec\_ L}^{\prime}}(i)}}} - \left( {\sum\limits_{i = 0}^{I - 1}{{Rec\_ L}^{\prime}(i)}} \right)^{2}} = {\frac{A\; 1}{A\; 2}\mspace{655mu}\left\lbrack {{Equation}\mspace{14mu} 6} \right\rbrack}}$$\mspace{79mu}{b = \frac{{\sum\limits_{i = 0}^{I - 1}{{Rec\_ c}(i)}} - {a \cdot {\sum\limits_{i = 0}^{I - 1}{{Rec\_ L}^{\prime}(i)}}}}{I}}$

In Equations 5 and 6, “I” is the number of neighboring pixels adjacentto an upper side and a left side of the chrominance block 2510 or theluminance block 2520. When sizes of the chrominance block 2510 and theluminance block 2520 are nS×nS as shown in FIGS. 25A and 25B, the number“I” is 2nS. Although upper and left pixels are used as neighboringpixels in FIGS. 25A and 25B, if right or lower neighboring pixels areprocessed and restored prior to a current block, the right or leftneighboring pixels may be used to obtain values of the parameters “a”and “b”. Also, in order to replace multiplication or subtraction with ashift operation, the number “I” of neighboring pixels may be anexponential power of 2. In general, since a value of ns which defines asize of a block is an exponential power of 2, the number “I” has a valueof an exponential power of 2. Values of “a′” and “b′” which are changedthrough up-scaling or rounding to reduce calculation complexity when avalue of the parameter “a” or “b” is obtained by using Equations 5 and 6may be used instead of the parameters “a” and “b”. Intra predictionmodes of FIGS. 25A and 25B may be defined as linear model intraprediction modes and may be used as one of intra prediction modes ofprediction units.

As described above with reference to FIGS. 18A through 25B, the numberof available intra prediction modes is not limited and an intraprediction mode applied to a prediction unit of a luminance component oran intra prediction mode applied to a prediction unit of a chrominancecomponent may be selectively used. For example, an intra prediction modecandidate group applied to a prediction unit of a luminance componentmay include 35 intra prediction modes including 33 intra predictionmodes using 33 (dx, dy) parameters shown in Table 3, a DC predictionmode, and a planar mode of FIG. 24. An intra prediction mode candidategroup applied to a prediction unit of a chrominance component is set tohave a smaller number of intra prediction modes than the intraprediction mode candidate group applied to the prediction unit of thechrominance component. For example, the intra prediction mode candidategroup of the chrominance component may include only 5 intra predictionmodes, that is, a vertical mode, a horizontal mode, a DC mode, a45°-diagonal mode, and an intra prediction mode of a luminance componentwhich is previously processed and determined. However, the number andtypes of intra prediction modes applied to prediction units of aluminance component and a chrominance component are not limited thereto.Also, various intra prediction modes other than the intra predictionmodes described with reference to FIGS. 18A through 25B may be includedin intra prediction mode candidates for intra prediction of achrominance component and a luminance component.

A process of encoding an intra prediction mode of a prediction unit ofan image will be explained in detail.

FIG. 26 is a block diagram illustrating an apparatus 2600 for encodingan intra prediction mode of an image, according to an exemplaryembodiment. FIG. 27 is a flowchart illustrating a method of encoding anintra prediction mode of an image, according to an exemplary embodiment.The apparatus 2600 may operate as the intra predictor 410 of theapparatus 400 of FIG. 4. An intra prediction mode encoder 2630 in FIG.26 may be included in the entropy encoder 450 of FIG. 4, instead of theintra predictor 410.

Referring to FIGS. 26 and 27, the apparatus 2600 includes a first intraprediction mode determiner 2610, a second intra prediction modedeterminer 2620, and an intra prediction mode encoder 2630.

In operation 2710, the first intra prediction mode determiner 2610determines an intra prediction mode having a minimum cost an intraprediction mode candidate group including a plurality of intraprediction modes as an intra prediction mode of a prediction unit of aluminance component by using intra prediction modes applicable to theprediction unit of the luminance component. For example, when an intraprediction mode candidate group applicable to the prediction unit of theluminance component includes 35 intra prediction modes including 33intra prediction modes using (dx, dy) parameters, a DC prediction mode,and a planar mode, the first intra prediction mode determiner 2610compares costs of error values obtained by using the 35 intra predictionmodes and determines an optimum intra prediction mode IntraMode_Lumaapplicable to a current prediction unit of a current luminancecomponent.

In operation 2720, the second intra prediction mode determiner 2620determines an intra prediction mode having a minimum cost in an intraprediction mode candidate group including a plurality of intraprediction modes as an intra prediction mode of a prediction unit of achrominance component by using intra prediction modes applicable to theprediction unit of the chrominance component. For example, when an intraprediction mode candidate group applicable to the prediction unit of thechrominance component includes 5 intra prediction modes, that is, aplanar mode, a vertical mode, a horizontal mode, a DC mode, and a45°-diagonal mode, the second intra prediction mode determiner 2620compares costs of error values obtained by using the intra predictionmodes and determines an optimum intra prediction mode IntraMode_Chromaapplicable to a current prediction unit of a current chrominancecomponent.

When an intra prediction mode of a luminance component which ispreviously processed and determined is included in an intra predictionmode candidate group of a chrominance component, the number of intraprediction modes of the chrominance component which are compared may bechanged according to whether an intra prediction mode of a chrominancecomponent which is same as an intra prediction mode of a luminancecomponent exists in the intra prediction mode candidate group of thechrominance component.

Also, only an intra prediction mode candidate group applicable to aprediction unit of a chrominance component basically includes a planarmode, a vertical mode, a horizontal mode, and a DC mode, and adetermined intra prediction mode of a prediction unit of a luminancecomponent corresponds to one of the planar mode, the vertical mode, thehorizontal mode, and the DC mode, an intra prediction mode candidategroup applicable to the prediction unit of the chrominance component maybe determined by replacing a mode which is same as the intra predictionmode of the prediction unit of the luminance component with a diagonalmode. In this case, the second intra prediction mode determiner 2620determines an optimum intra prediction mode IntraMode_Chroma by usingthe intra prediction modes of the intra prediction mode candidate group.

As described above, an intra prediction mode candidate group applied toa prediction unit of a chrominance component may include separately aspecific intra prediction mode which is not applied to a prediction unitof a luminance component. For example, an intra prediction modecandidate group of a luminance component may not include an LM mode, andonly an intra prediction mode candidate group of a chrominance componentmay include an LM mode. The second intra prediction mode determiner 2620may determine an optimum intra prediction mode IntraMode_Chroma byapplying each intra prediction mode in the intra prediction modecandidate group including the LM mode to the prediction unit of thechrominance component.

The determined intra prediction mode of the prediction unit of thechrominance component and the determined intra prediction mode of theprediction unit of the luminance component need to be signaled to adecoding side. Accordingly, the intra prediction mode encoder 2630encodes final intra prediction modes applied to the prediction unit ofthe chrominance component and the prediction unit of the luminancecomponent. In particular, in operation 2730, when the intra predictionmode of the prediction unit of the luminance component is the same asone of predetermined intra prediction modes included in an intraprediction mode candidate group of the chrominance component, the intraprediction mode encoder 2630 reconstructs the intra prediction modecandidate group of the prediction unit of the chrominance component toreduce redundancy of the intra prediction mode included in the intraprediction mode candidate group applied to the prediction unit of thechrominance component which is same as the intra prediction mode of theprediction unit of the luminance component. In operation 2740, the intraprediction mode encoder 2630 encodes the intra prediction mode of theprediction unit of the chrominance component determined by the secondintra prediction mode determiner 2620 based on the reconstructed intraprediction mode candidate group of the prediction unit of thechrominance component. The intra prediction mode encoder 2630 encodesdifferently intra prediction mode information of the chrominancecomponent according to whether the intra prediction mode of theprediction unit of the luminance component is the same as the intraprediction mode of the prediction unit of the chrominance component andwhether the intra prediction mode of the prediction unit of theluminance component is the same as one of the predetermined intraprediction modes included in the intra prediction mode candidate groupof the chrominance component as well as an index indicating the intraprediction mode of the prediction unit of the luminance component.

In detail, when the intra prediction mode of the prediction unit of theluminance component is the same as the intra prediction mode of theprediction unit of the chrominance component, the intra prediction modeencoder 2630 encodes only a flag DM flag indicating whether the intraprediction mode of the prediction unit of the chrominance component isthe same as the intra prediction mode of the prediction unit of theluminance component as intra prediction mode information about theprediction unit of the chrominance component. For example, when thedetermined intra prediction mode of the prediction unit of the luminancecomponent and the determined intra prediction mode of the predictionunit of the chrominance component are horizontal modes H, the intraprediction mode encoder 2630 encodes an index and a flag DM flagindicating the intra prediction mode of the prediction unit of theluminance component as intra prediction mode information.

Assuming that the intra prediction mode of the prediction unit of theluminance component is not the same as the intra prediction mode of theprediction unit of the chrominance component and is the same as one ofthe predetermined intra prediction modes included in the intraprediction mode candidate group of the chrominance component, when theintra prediction mode candidate group of the chrominance component isreconstructed, the intra prediction mode encoder 2630 encodes an indexindicating the corresponding intra prediction mode in the reconstructedintra prediction mode candidate group and a flag DM flag.

Assuming that the intra prediction mode of the prediction unit of theluminance component and the intra prediction mode of the prediction unitof the chrominance component are not the same and the intra predictionmode of the prediction unit of the luminance component is not the sameas any one of the predetermined intra prediction modes included in theintra prediction mode candidate group of the chrominance component, theintra prediction mode encoder 2630 encodes an index indicating thecorresponding intra prediction mode in the initially set intraprediction mode candidate group and a flag DM flag.

If a specific intra prediction mode, for example, the LM mode, which isnot applied to the prediction unit of the luminance component in theintra prediction mode candidate group applied to the prediction unit ofthe chrominance component is applied to only the chrominance component,a flag (LM flag) indicating whether the intra prediction mode of theprediction unit of the chrominance component is the LM mode may beincluded as intra prediction mode information about the prediction unitof the chrominance component. When the LM mode is included as theprediction unit of the chrominance component and the intra predictionmode of the prediction unit of the chrominance component is the LM mode,the intra prediction mode encoder 2630 encodes only a DM flag and an LMflag.

When the intra prediction mode of the prediction unit of the luminancecomponent and the intra prediction mode of the prediction unit of thechrominance component are not the same, and the intra prediction mode ofthe prediction unit of the chrominance component is not the LM mode, theintra prediction mode encoder 2630 encodes index information indicatingthe intra prediction mode of the prediction unit of the chrominancecomponent, the DM flag, and the LM flag. In this case, assuming that theintra prediction mode of the prediction unit of the luminance componentis the same as one of the predetermined intra prediction modes includedin the intra prediction mode candidate group of the chrominancecomponent, when the intra prediction mode candidate group of thechrominance component is reconstructed, the intra prediction modeencoder 2630 encodes the LM flag, the DM flag and the index indicatingthe corresponding intra prediction mode in the reconstructed intraprediction mode candidate group.

A process of encoding an intra prediction mode of a prediction unit of achrominance component will be explained in detail with reference to FIG.28.

FIG. 28 is a flowchart illustrating a process of encoding an intraprediction mode of an image, according to an exemplary embodiment.

Referring to FIGS. 26 and 28, in operation 2810, the first intraprediction mode determiner 2610 and the second intra prediction modedeterminer 2620 determine intra prediction modes of a prediction unit ofa chrominance component and a prediction unit of a luminance componentby using a plurality of available intra prediction modes. As describedabove, it is assumed that an intra prediction mode candidate groupapplicable to a prediction unit of a luminance component includes 35intra prediction modes including 33 intra prediction modes using (dx,dy) parameters, a DC prediction mode, and a planar mode, and an intraprediction candidate group applicable to a prediction unit of achrominance component includes 5 intra prediction units including aplanar mode, a vertical mode, a horizontal mode, a DC mode, and a45°-diagonal mode. As described above, an intra prediction modeapplicable to a chrominance component may include an intra predictionmode (derived mode, hereinafter, referred to as a “DM mode”) of aluminance component which is previously processed and determined. The DMmode is the same as IntraMode_Luma. The intra prediction mode encoder2630 assigns a predetermined code word to each of the 35 intraprediction modes applicable to the prediction unit of the luminancecomponent, and encodes a code word corresponding to the determined intraprediction mode IntraMode Luma of the prediction unit of the luminancecomponent. An Exp-Golomb (Exponential Goloms) code may be used as a codeword.

In operation 2820, the intra prediction mode encoder 2630 determineswhether the determined intra prediction mode IntraMode_Luma of theprediction unit of the luminance component is the same as thecorresponding intra prediction mode IntraMode_Chroma of the predictionunit of the chrominance component. If the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component is thesame as the corresponding intra prediction mode IntraMode_Chroma of theprediction unit of the chrominance component, the method proceeds tooperation 2830. In operation 2830, the intra prediction mode encoder2630 sets to 0 a flag DM flag indicating whether the intra predictionmode of the prediction unit of the chrominance component is the same asthe intra prediction mode of the prediction unit of the luminancecomponent. When the flag DM flag is 0, it means that the intraprediction mode of the prediction unit of the luminance component is thesame as the prediction mode of the chrominance component. When the flagDM flag is 1, it means that the intra prediction mode of the predictionunit of the luminance component is different from the intra predictionmode of the prediction unit of the chrominance component. A value of theflag DM flag may be set in an opposite way. As such, assuming that theflag DM flag is 0, that is, when the intra prediction mode of theprediction unit of the luminance component is the same as the predictionmode of the chrominance component, when only the flag DM flag is encodedand transmitted to a decoding side and the decoding side receives theflag DM flag having a value of 0, the corresponding intra predictionmode of the prediction unit of the chrominance component may bedetermined from the intra prediction mode of the prediction unit of theluminance component which is first decoded. For example, when the intraprediction modes of the prediction unit of the chrominance component andthe prediction unit of the luminance component are horizontal modes, theintra prediction mode encoder 2630 encodes only the flag DM flag andintra prediction mode information IntraMode_Luma of the prediction unitof the luminance component.

When it is determined in operation 2820 that the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component is notthe same as the corresponding intra prediction mode IntraMode_Chroma ofthe prediction unit of the chrominance component, the intra predictionmode encoder 2630 has to encode a separate intra prediction mode appliedto the prediction unit of the chrominance component. In operation 2840,the intra prediction mode encoder 2630 determines whether the intraprediction mode IntraMode_Luma of the prediction unit of the luminancecomponent is included in predetermined intra prediction modesCandidate_Modes in an intra prediction mode candidate group of theprediction unit of the luminance component. For example, when the intraprediction mode candidate group applicable to the prediction unit of thechrominance component includes 5 intra prediction modes including aplanar mode, a vertical mode, a horizontal mode, a DC mode, and a45°-diagonal mode, the intra prediction mode encoder 2630 determineswhether the intra prediction mode IntraMode_Luma of the prediction unitof the luminance component corresponds to one of the 5 intra predictionmodes. Alternatively, when the intra prediction mode candidate groupapplicable to the prediction unit of the chrominance component basicallyincludes a planar mode, a vertical mode, a horizontal mode, and a DCmode, and uses a diagonal mode instead of a mode which is same as theintra prediction mode of the prediction unit of the luminance componentonly when the intra prediction mode of the prediction unit of theluminance component corresponds to one of the planar mode, the verticalmode, the horizontal mode, and the DC mode, the intra prediction modeencoder 2630 determines whether the intra prediction mode of theprediction unit of the luminance component corresponds to one of theplanar mode, the vertical mode, the horizontal mode, and the DC mode.

When it is determined in operation 2840 that the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component is notincluded in the predetermined intra prediction modes Candidate_Modes,the method proceeds to operation 2850. In operation 2850, the intraprediction mode encoder 2630 encodes the intra prediction mode of theprediction unit of the chrominance component by using a second intraprediction mode candidate group applied to the present component whichis preset. For example, a predetermined index may be preset as shown inTable 6 in each of intra prediction modes included in the second intraprediction mode candidate group applied to the chrominance component.

TABLE 6 Intra prediction mode of chrominance component Code word Planarmode 0 Vertical mode 10 Horizontal mode 110 DC mode 1110 Diagonal mode1111

When it is determined that the intra prediction mode IntraMode_Luma ofthe prediction unit of the luminance component is not included in thepredetermined intra prediction modes Candidate_Modes, the intraprediction mode encoder 2630 encodes the intra prediction mode of theprediction unit of the chrominance component by using the predeterminedindex as shown in Table 6. For example, when the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component is notincluded in the predetermined intra prediction modes Candidate_Modesbecause the intra prediction mode IntraMode_Luma of the prediction unitof the luminance component is a prediction mode which has adirectionality using (−13,32) as (dx, dy) as shown in Table 3, the intraprediction mode encoder 2630 encodes the intra prediction mode of theprediction unit of the chrominance component by using the predeterminedindex as shown in Table 6. The code word, number and types of intraprediction modes of chrominance component may be changed.

When it is determined in operation 2840 that the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component isincluded in the predetermined intra prediction modes Candidate_Modes,the method proceeds to operation 2860. In operation 2860, the intraprediction mode encoder 2630 reconstructs an intra prediction modecandidate group applied to the prediction unit of the chrominancecomponent to remove redundancy of the intra prediction mode included inthe intra prediction mode candidate group of the prediction unit of thechrominance component which is same as the intra prediction mode of theprediction unit of the luminance component. For example, when the intraprediction mode IntraMode_Luma of the prediction unit of the luminancecomponent is a DC mode, the intra prediction mode IntraMode_Chroma ofthe prediction unit of the chrominance component is a horizontal mode,and the predetermined intra prediction modes Candidate_Modes are aplanar mode, a vertical mode, a horizontal mode, and a DC mode, the DCmode which is the intra prediction mode IntraMode_Luma of the predictionunit of the luminance component corresponds to one of the predeterminedintra prediction modes Candidate_Modes applicable to the chrominancecomponent. In this case, since the DC mode may be obtained from theintra prediction mode of the prediction unit of the luminance componentin Table 6, a separate code word does not need to be assigned.Accordingly, according to the present exemplary embodiment, when theintra prediction mode IntraMode_Luma of the prediction unit of theluminance component is included in the predetermined intra predictionmodes Candidate_Modes of the prediction unit of the luminance component,since the intra prediction mode which is similar to the intra predictionmode of the prediction unit of the luminance component and is includedin the intra prediction mode candidate group of the prediction unit ofthe chrominance component may be replaced with an intra prediction modeother than the predetermined intra prediction modes Candidate_Modes orthe redundant intra prediction mode may be signaled through the DM flag,the second intra prediction mode candidate group may be reconstructed byremoving the redundant intra prediction mode of the chrominancecomponent and an index indicating each intra prediction mode may beassigned based on the reconstructed second intra prediction modecandidate group.

In detail, when the intra prediction mode IntraMode_Luma of theprediction unit of the luminance component corresponds to one of thepredetermined intra prediction modes Candidate_Modes applicable to thechrominance component, the intra prediction mode encoder 2630 mayreplace the prediction mode included in the intra prediction modecandidate group of the chrominance with an intra prediction mode notincluded in the predetermined intra prediction modes Candidate_Modes.For example, when the intra prediction mode candidate group applied tothe prediction unit of the chrominance component basically includes aplanar mode, a vertical mode, a horizontal mode, and a DC mode, and thedetermined intra prediction mode of the prediction unit of the luminancecomponent corresponds to one of the planar mode, the vertical mode, thehorizontal mode, and the DC mode, a mode which is same as the intraprediction mode of the prediction unit of the luminance component may bereplaced with a diagonal mode. It is assumed that a code word isassigned as shown in Table 7 only to the predetermined intra predictionmodes Candidate_Modes, that is, a planar mode, a vertical mode, ahorizontal mode, and a DC mode, in the intra prediction mode candidategroup of the prediction unit of the chrominance component.

TABLE 7 Intra prediction mode of chrominance component Code word Planarmode 0 Vertical mode 10 Horizontal mode 110 DC mode 111

When the DC mode which is the intra prediction mode IntraMode_Luma ofthe prediction unit of the luminance component is same as the DC modefrom among the predetermined intra prediction modes Candidate_Modesapplicable to the chrominance component, the DC mode is replaced with adiagonal mode, and Table 7 is reconstructed as Table 8.

TABLE 8 Intra prediction mode of chrominance component Code word Planarmode 0 Vertical mode 10 Horizontal mode 110 Diagonal mode 111

Also, when the intra prediction mode IntraMode_Luma of the predictionunit of the luminance component corresponds to one of the predeterminedintra prediction modes Candidate_Modes applicable to the prediction unitof the chrominance component, the intra prediction mode encoder 2630 mayremove a redundant intra prediction mode which is redundant in the intraprediction mode candidate group of the chrominance component. Forexample, when the intra prediction mode candidate group applicable tothe prediction unit of the chrominance component includes 5 intraprediction modes including a planar mode, a vertical mode, a horizontalmode, a DC mode, and a 45°-diagonal mode, and the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component is adiagonal mode, indices indicating the intra prediction mode candidategroup of the chrominance component may remove the diagonal mode in Table6 and may be reconstructed as shown in Table 9. In this case, since thenumber of intra prediction modes included in the intra prediction modecandidate group of the chrominance component is reduced, the number ofbits which is a value of an index indicating each intra prediction modemay be reduced.

TABLE 9 Intra prediction mode of chrominance component Code word Planarmode 0 Vertical mode 10 Horizontal mode 110 DC mode 111

The encoding mode predictor 2630 may reduce the number of bits necessaryto encode an intra prediction mode by assigning a code word only to(n−1) intra prediction modes in an intra prediction mode candidate groupincluding n (n is an integer) intra prediction modes of the chrominancecomponent, replacing an intra prediction mode which is same as an intraprediction mode of the prediction unit of the luminance component, andremoving a redundant code word.

Also, the encoding mode predictor 2630 may not perform indexing byexcluding the intra prediction mode which is same as the intraprediction mode of the luminance component in the intra prediction modecandidate group of the chrominance component which is preset from theintra prediction mode candidate group.

When the number of available intra prediction modes in the intraprediction mode candidate group of the chrominance component is n (n isa positive integer), the intra prediction encoder 2630 reconstructs theintra prediction mode candidate group of the chrominance component byreplacing an intra prediction mode of the chrominance component which issame as the intra prediction mode of the luminance component from amongthe (n−1) predetermined intra prediction modes Candidate_Modes which arepreset with a remaining one intra prediction mode.

Alternatively, operation 2860 in which the intra prediction modecandidate group of the prediction unit of the chrominance component maybe performed by excluding an intra prediction mode of the chrominancecomponent which is same as the intra prediction mode of the luminancecomponent and assigning a code word only to remaining intra predictionmodes. For example, when the intra prediction mode IntraMode_Luma of theprediction unit of the luminance component is a DC mode, the intraprediction mode IntraMode_Chroma of the prediction unit of thechrominance component is a horizontal mode, the predetermined intraprediction modes Candidate_Modes are a vertical mode, a horizontal mode,a DC mode, and a DM mode, the DC mode which is the intra prediction modeIntraMode_Luma of the prediction unit of the luminance componentcorresponds to one of the predetermined intra prediction modesCandidate_Modes applicable to the chrominance component. In this case,when a separate code word is assigned although the DC mode in Table 6may be obtained from the intra prediction mode of the prediction unit ofthe luminance component, the number of bits used to encode a predictionmode may be increased by assigning a separate code word to a redundantprediction mode. Accordingly, it is preferable that a code word isassigned only to intra prediction modes other than the DC mode. That is,the intra prediction mode encoder 2630 may encode the intra predictionmode of the chrominance component by assigning a code word only to 4intra prediction modes other than the DC mode in Table 9.

Referring back to FIG. 28, in operation 2870, the intra prediction modeencoder 2630 encodes the intra prediction mode of the chrominancecomponent by using the reconstructed intra prediction mode candidategroup of the prediction unit of the chrominance component. When theintra prediction mode candidate group of the chrominance component isreconstructed, the intra prediction mode encoder 2630 encodes a codeword corresponding to the intra prediction mode of the prediction unitof the chrominance component as intra prediction mode information aboutthe prediction unit of the chrominance component.

A process of decoding an intra prediction mode of a prediction unit ofan image will be explained in detail.

FIG. 29 is a block diagram illustrating an apparatus 2900 for decodingan intra prediction mode of an image, according to an exemplaryembodiment. FIG. 30 is a flowchart illustrating a method of decoding anintra prediction mode of an image, according to an exemplary embodiment.The apparatus 2900 may operate as the intra predictor 550 of theapparatus 500 of FIG. 5. In FIG. 29, an intra prediction mode obtainer2910 may be included in the parser 510 or the entropy decoder 520 ofFIG. 5, instead of the intra predictor 550.

Referring to FIGS. 29 and 30, the apparatus 2900 includes the intraprediction mode obtainer 2910, a first intra prediction performer 2920,and a second intra prediction performer 2930.

In operation 3010, the intra prediction mode obtainer 2910 obtains intraprediction mode information IntraMode_Luma of a prediction unit of aluminance component constituting an image from a bitstream. Also, theintra prediction mode obtainer 2910 obtains a flag DM flag indicatingwhether the intra prediction mode IntraMode_Chroma of the predictionunit of the chrominance component corresponding to a prediction unit ofa luminance component is the same as the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component asintra prediction mode information of the prediction unit of thechrominance component.

In operation 3020, when it is determined that the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component is thesame as one of the predetermined intra prediction modes Candidate_Modesincluded in an intra prediction mode candidate group applicable to theprediction unit of the chrominance component, the intra prediction modeobtainer 2910 reconstructs the intra prediction mode candidate group ofthe chrominance component to remove redundancy of the intra predictionmode included in the intra prediction mode candidate group of thechrominance component which is same as the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component.

In operation 3030, the intra prediction mode obtainer 2910 determinesthe intra prediction mode of the prediction unit of the chrominancecomponent by using an index indicating one of the intra prediction modesincluded in the reconstructed intra prediction mode candidate group ofthe chrominance component which is obtained from the bitstream.

When the intra prediction mode obtainer 2910 determines the intraprediction modes of the prediction unit of the chrominance component andthe prediction unit of the luminance component, the first intraprediction performer 2920 generates and outputs a prediction value ofthe prediction unit of the luminance component by performing intraprediction on the prediction unit of the luminance component, and thesecond intra prediction performer 2930 generates and outputs aprediction value of the prediction unit of the chrominance component byperforming intra prediction on the prediction unit of the chrominancecomponent.

FIG. 31 is a flowchart illustrating a process of decoding an intraprediction mode of an image, according to an exemplary embodiment.

Referring to FIGS. 29 and 31, in operation 3110, the intra predictionmode obtainer 2910 obtains a flag DM indicating whether the intraprediction mode IntraMode_Luma of the prediction unit of the chrominancecomponent and the intra prediction mode of the prediction unit of theluminance component are the same as the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component fromthe bitstream.

In operation 3120, the intra prediction mode obtainer 2910 determineswhether the intra prediction mode IntraMode_Chroma of the predictionunit of the chrominance component is the same as the intra predictionmode IntraMode_Luma of the prediction unit of the luminance componentbased on the flag DM.

When it is determined in operation 3120 that a value of the flag DM is0, that is, when the intra prediction mode obtainer 2910 determines thatthe intra prediction mode IntraMode_Chroma of the prediction unit of thechrominance component is the same as the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component, inoperation 3130, the intra prediction mode obtainer 2910 sets the intraprediction mode of the prediction unit of the chrominance component tothe intra prediction mode of the prediction unit of the luminancecomponent. As such, when the flag DM is 0, that is, when the intraprediction mode of the prediction unit of the luminance component is thesame as the prediction mode of the chrominance component, the intraprediction mode of the chrominance component may be determined by usingonly the flag DM.

When it is determined in operation 3120 that the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component is notthe same as the corresponding the intra prediction mode IntraMode_Chromaof the prediction unit of the chrominance component, the intraprediction mode obtainer 2910 determines whether the intra predictionmode IntraMode_Luma of the prediction unit of the luminance component isincluded in the predetermined intra prediction modes Candidate_Modes inthe intra prediction mode candidate group applied to the prediction unitof the chrominance component. For example, when the intra predictionmode candidate group applicable to the prediction unit of thechrominance component includes 5 intra prediction modes including aplanar mode, a vertical mode, a horizontal mode, a DC mode, and a45°-diagonal mode, the intra prediction mode obtainer 2910 determineswhether the intra prediction mode IntraMode_Luma of the prediction unitof the luminance component corresponds to one of the 5 intra predictionmodes. Alternatively, when the intra prediction mode candidate groupapplied to the prediction unit of the chrominance component basicallyincludes a planar mode, a vertical mode, a horizontal mode, and a DCmode, and a diagonal mode instead of a mode which is same as the intraprediction mode of the prediction unit of the luminance component isused only when the intra prediction mode of the prediction unit of theluminance component corresponds to only one of the predetermined intraprediction modes Candidate_Modes, for example, the planar mode, thevertical mode, the horizontal mode, and the DC mode, the intraprediction mode obtainer 2910 determines whether the intra predictionmode of the prediction unit of the luminance component corresponds toone of the planar mode, the vertical mode, the horizontal mode, and theDC mode. When it is determined in operation 3140 that the intraprediction mode IntraMode_Luma of the prediction unit of the luminancecomponent is not included in the predetermined intra prediction modesCandidate_Modes, the method proceeds to operation 3150. In operation3150, the intra prediction mode obtainer 2910 reads an index from thebitstream and determines the intra prediction mode of the chrominancecomponent indicated by the read index. As described above, apredetermined code word may be preset as shown in Table 6 for intraprediction modes applied to the chrominance component, and the intraprediction mode obtainer 2910 may read from the bitstream the code word,that is, a value of an index, and may determine the intra predictionmode of a prediction unit of the chrominance component.

When it is determined in operation 3140 that the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component isincluded in the predetermined intra prediction modes Candidate_Modes,the method proceeds to operation 3160. In operation 3160, the intraprediction mode obtainer 2910 reconstructs the intra prediction modecandidate group applied to the prediction unit of the chrominancecomponent to remove redundancy of the intra prediction mode included inthe intra prediction mode candidate group of the prediction unit of thechrominance component which is same as the intra prediction mode of theprediction unit of the luminance component. As described above, when theintra prediction mode IntraMode_Luma of the prediction unit of theluminance component is a DC mode, the intra prediction modeIntraMode_Chroma of the prediction unit of the chrominance component isa horizontal mode, and the predetermined intra prediction modesCandidate_Modes are a vertical mode, a horizontal mode, a DC mode and aDM mode, the DC mode which is the intra prediction mode IntraMode_Lumaof the prediction unit of the luminance component corresponds to one ofthe predetermined intra prediction modes Candidate_Modes applicable tothe chrominance component. In this case, since the DC mode in Table 6may be obtained from the intra prediction mode of the prediction unit ofthe luminance component, a separate code word does not need to beassigned. Accordingly, when the intra prediction mode IntraMode_Luma ofthe prediction unit of the luminance component is included in thepredetermined intra prediction modes Candidate_Modes of the predictionunit of the chrominance component, the intra prediction mode obtainer2910 may reconstruct a second intra prediction mode candidate group byreplacing the intra prediction mode which is same as the intraprediction mode of the prediction unit of the luminance component and isincluded in the intra prediction mode candidate group of the predictionunit of the chrominance component with an intra prediction mode otherthan the predetermined intra prediction modes Candidate_Modes orremoving the redundant intra prediction mode of the chrominancecomponent, and may determine an index indicating each intra predictionmode based on the reconstructed second intra prediction mode candidategroup. A process of reconstructing the intra prediction mode candidategroup of the prediction unit of the chrominance component may be thesame as the aforesaid reconstruction method performed at an encodingside.

In detail, when the DC mode which is the intra prediction modeIntraMode_Luma of the prediction unit of the luminance componentcorresponds to one of the predetermined intra prediction modesCandidate_Modes applicable to the chrominance component, the intraprediction mode obtainer 2910 may replace the prediction mode includedin the intra prediction mode candidate group of the chrominancecomponent with an intra prediction mode not included in thepredetermined intra prediction modes Candidate_Modes. For example, whenthe intra prediction mode candidate group applied to the prediction unitof the chrominance component basically includes a planar mode, avertical mode, a horizontal mode, and a DC mode, and the determinedintra prediction mode of the prediction unit of the luminance componentcorresponds to one of the planar mode, the vertical mode, the horizontalmode, and the DC mode, a mode which is same as the intra prediction modeof the prediction unit of the luminance component may be replaced with adiagonal mode. Assuming that a code word is assigned as shown in Table 7to the predetermined intra prediction modes Candidate_Modes, that is,the vertical mode, the horizontal mode, the DC mode, and the DM mode, inthe intra prediction mode candidate group of the prediction unit of thechrominance component, when the DC mode which is the intra predictionmode of the prediction unit of the luminance component is same as the DCmode of the predetermined intra prediction modes Candidate_Modesapplicable to the chrominance component, the DC mode is replaced with adiagonal mode and is reconstructed as shown in Table 8. The intraprediction mode obtainer 2910 may determine whether an index of theintra prediction mode included in the bitstream indicates which intraprediction mode by using the reconstructed intra prediction modecandidate group.

Also, when the intra prediction mode IntraMode_Luma of the predictionunit of the luminance component corresponds to one of the predeterminedintra prediction modes Candidate_Modes applicable to the prediction unitof the chrominance component, the intra prediction mode obtainer 2910may remove an intra prediction mode which is redundant in the intraprediction mode candidate group of the chrominance component. Forexample, when the intra prediction mode candidate group applicable tothe prediction unit of the chrominance component includes 5 intraprediction modes including a planar mode, a vertical mode, a horizontalmode, a DC mode, and a 45°-diagonal mode, and the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component is adiagonal mode, indices indicating the intra prediction mode candidategroup of the chrominance component may be reconstructed as shown inTable 9 by excluding the diagonal mode in Table 6. The intra predictionmode obtainer 2910 may determine whether the index of the intraprediction mode of the chrominance component included in the bitstreamindicates which intra prediction mode by using the reconstructed intraprediction mode candidate group of the chrominance component. Theencoding mode obtainer 2910 may reduce the number of bits necessary todecode an intra prediction mode by assigning a code word only to (n−1)intra prediction modes in an intra prediction mode candidate groupincluding n (n is an integer) intra prediction modes of the chrominancecomponent and replacing an intra prediction mode which is same as theintra prediction mode of the luminance component with one remainingintra prediction mode to remove a redundant code word. As describedabove, when the DC mode which is the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component is sameas the DC mode of the predetermined intra prediction modesCandidate_Modes applicable to the chrominance component, a code word ofthe DC mode may be used as a code word indicating the diagonal mode. Assuch, when the number of available intra prediction modes in the intraprediction mode candidate group of the chrominance component is n (n isa positive integer), the intra prediction mode obtainer 2910reconstructs the intra prediction mode candidate group of thechrominance component by replacing an intra prediction of thechrominance component which is same as the intra prediction mode of theluminance component from among the preset (n−1) predetermined intraprediction modes Candidate_Modes with one remaining intra predictionmode.

Alternatively, operation 3160 in which the intra prediction modecandidate group of the prediction unit of the chrominance component isreconstructed may be performed by assigning a code word only to intraprediction modes other than the intra prediction mode of the chrominancecomponent which is the same as the intra prediction mode of theluminance component. For example, when the intra prediction modeIntraMode_Luma of the prediction unit of the luminance component is a DCmode, the intra prediction mode IntraMode_Chroma of the prediction unitof the chrominance component is a horizontal mode, and the predeterminedintra prediction modes Candidate_Modes are a vertical mode, a horizontalmode, a DC mode, and a DM mode, the DC mode which is the intraprediction mode IntraMode_Luma of the prediction unit of the luminancecomponent corresponds to one of the predetermined intra prediction modesCandidate_Modes applicable to the chrominance component. In this case,since the DC mode in Table 6 may be obtained from the intra predictionmode of the prediction unit of the luminance component, a code word isassigned only to intra prediction modes other than the DC mode. That is,the intra prediction mode candidate group is reconstructed by assigninga code word only to remaining 4 intra prediction modes other than the DCmode as shown in Table 8.

Referring back to FIG. 31, in operation 3170, the intra prediction modeobtainer 2910 decodes the intra prediction mode of the chrominancecomponent by using the reconstructed intra prediction mode candidategroup of the prediction unit of the chrominance component and the indexobtained from the bitstream. When the intra prediction mode candidategroup of the chrominance component is reconstructed, the index indicatesinformation (code word) about the intra prediction mode applied to thecurrent prediction unit of the chrominance component in thereconstructed intra prediction mode candidate group of the chrominancecomponent through the same process as that performed on the encodingside, and the intra prediction mode obtainer 2910 determines the intraprediction mode of the prediction unit of the chrominance component byselecting the intra prediction mode indicated by the index from theintra prediction mode candidate group of the prediction unit of thechrominance component which is reconstructed in the same manner as thatof the encoding side.

The exemplary embodiments may be embodied as computer-readable codes ona computer-readable recording medium. The computer-readable recordingmedium includes any storage device that may store data which may be readby a computer system. Examples of the computer-readable recording mediuminclude read-only memories (ROMs), random-access memories (RAMs),CD-ROMs, magnetic tapes, floppy disks, and optical data storage devices.The computer-readable recording medium may be distributed overnetwork-coupled computer systems so that the computer-readable codes arestored and executed in a distributed fashion.

While the exemplary embodiments has been particularly shown anddescribed with reference to exemplary embodiments thereof by usingspecific terms, the embodiments and terms have merely been used toexplain the exemplary embodiments and should not be construed aslimiting the scope of the exemplary embodiments as defined by theclaims. The exemplary embodiments should be considered in a descriptivesense only and not for purposes of limitation. Therefore, the scope ofthe exemplary embodiments is defined not by the detailed description ofthe exemplary embodiments but by the appended claims, and alldifferences within the scope will be construed as being included in theexemplary embodiments.

The invention claimed is:
 1. A method of decoding an image, the methodcomprising: splitting the image into a plurality of maximum codingunits; hierarchically splitting a maximum coding unit from among theplurality of maximum coding units into at least one coding unitincluding a current coding unit based on split information; obtainingintra prediction mode information regarding a prediction unit of a firstimage component of the image from a bitstream; when an intra predictionmode indicated by the intra prediction mode information regarding theprediction unit of the first image component is one of a planar mode, aDC mode, a horizontal mode and a vertical mode, obtaining an intraprediction mode candidate group regarding a second image component,wherein the intra prediction mode candidate group regarding the secondimage component includes a diagonal mode; obtaining intra predictionmode information regarding a prediction unit of the second imagecomponent; determining an intra prediction mode regarding the predictionunit of the second image component from among a plurality of intraprediction modes which are included in the intra prediction modecandidate group regarding the second image component based on theobtained intra prediction mode information regarding the prediction unitof the second image component; performing intra prediction on theprediction unit of the first image component according to the intraprediction mode indicated by the intra prediction mode informationregarding the prediction unit of the first image component; andperforming the intra prediction on the prediction unit of the secondimage component according to the determined intra prediction mode,wherein the performing the intra prediction on the prediction unit ofthe first image component comprises, when the intra prediction modeindicated by the intra prediction mode information regarding theprediction unit of the first image component is the planar mode,obtaining a prediction value of a current pixel included in a block inthe prediction unit of the first image component using four neighboringpixels of the block in the prediction unit of the first image component,wherein the current coding unit is a coding unit which is not splitaccording to the split information, wherein the prediction unit of thefirst image component is determined by splitting the current coding unitbased on partition type information, wherein the partition typeinformation indicates whether a partition type of symmetricallysplitting the current coding unit along at least one of a height and awidth of the current coding unit, and wherein the first image componentis a luma component and the second image component is a chromacomponent.
 2. The method of claim 1, wherein the intra prediction modeinformation regarding the prediction unit of the second image componentcomprises an index which indicates one of the plurality of intraprediction modes which are included in the intra prediction modecandidate group regarding the second image component.
 3. A method ofencoding an image, the method comprising: determining a plurality ofmaximum coding units by splitting the image into a plurality of themaximum coding units; determining at least one coding unit including acurrent coding unit by hierarchically splitting a maximum coding unitfrom among the plurality of maximum coding units into the at least onecoding unit, wherein the current coding unit is a coding unit of acurrent depth which is not split into coding units of a lower depth;determining one or more prediction units including a prediction unit ofa first image component by splitting the current coding unit;determining an intra prediction mode regarding the prediction unit ofthe first image component by performing intra prediction on theprediction unit of the first image component; determining an intraprediction mode regarding a prediction unit of a second image componentby performing the intra prediction on the prediction unit of the secondimage component; generating intra prediction mode information regardingthe prediction unit of the first image component of the image; when anintra prediction mode indicated by the intra prediction mode informationregarding the prediction unit of the first image component is one of aplanar mode, a DC mode, a horizontal mode and a vertical mode,generating an intra prediction mode candidate group regarding the secondimage component, wherein the intra prediction mode candidate groupregarding the second image component includes a diagonal mode;generating intra prediction mode information regarding a prediction unitof the second image component based on a plurality of intra predictionmodes included in the intra prediction mode candidate group regardingthe second image component; generating split information which indicateswhether a split of coding units in the maximum coding unit, generatingpartition type information which indicates whether a partition type ofsymmetrically splitting the current coding unit along at least one of aheight and a width of the current coding unit; and generating abitstream including the intra prediction mode information regarding theprediction unit of the first image component of the image, the intraprediction mode information regarding the prediction unit of the secondimage component, the split information and the partition typeinformation, wherein the determining of the intra prediction moderegarding the prediction unit of the first image component compriseswhen a prediction value of a current pixel included in a block in theprediction unit of the first image component is obtained by using fourneighboring pixels of the block in the prediction unit of the firstimage component in performing the intra prediction on the predictionunit of the first image component, determining the intra prediction moderegarding the prediction unit of the first image component as the planarmode, and wherein the first image component is a luma component and thesecond image component is a chroma component.
 4. A non-transitorycomputer-readable storage medium storing a bitstream comprising: intraprediction mode information regarding a prediction unit of a first imagecomponent of an image; intra prediction mode information regarding aprediction unit of a second image component of the image; splitinformation which indicates whether a split of coding units in a maximumcoding unit; and partition type information which indicates whether apartition type of symmetrically splitting a coding unit of a first depthalong at least one of a height and a width of the coding unit of thefirst depth when the coding unit of the first depth is not split intocoding units of a second depth which is lower than the first depth,wherein a plurality of maximum coding units are determined by splittingthe image into a plurality of maximum coding units, at least one codingunit including a current coding unit is determined by hierarchicallysplitting a maximum coding unit from among the plurality of maximumcoding units into the at least one coding unit, wherein the currentcoding unit is a coding unit of a current depth which is not split intocoding units of a lower depth, wherein one or more prediction unitsincluding the prediction unit of the first image component aredetermined by splitting the current coding unit, when an intraprediction mode indicated by the intra prediction mode informationregarding the prediction unit of the first image component is one of aplanar mode, a DC mode, a horizontal mode and a vertical mode, an intraprediction mode candidate group regarding the second image component isgenerated, wherein the intra prediction mode candidate group regardingthe second image component includes a diagonal mode, the intraprediction mode information regarding the prediction unit of the secondimage component is generated based on a plurality of intra predictionmodes included in the intra prediction mode candidate group regardingthe second image component, the partition type information is generatedto indicate whether a partition type of symmetrically splitting thecurrent coding unit along at least one of a height and a width of thecurrent coding unit, and wherein when a prediction value of a currentpixel included in a block in the prediction unit of the first imagecomponent is obtained by using four neighboring pixels of the block inthe prediction unit of the first image component in performing intraprediction on the prediction unit of the first image component, theintra prediction mode regarding the prediction unit of the first imagecomponent is determined as the planar mode, and wherein the first imagecomponent is a luma component and the second image component is a chromacomponent.